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Q. What are Most Frequently Asked Questions?
A.
· How do I know if my Server cluster is a supported configuration?
· How many servers can there be in a single cluster?
· How do I benefit from more than two nodes in a cluster?
· Are dynamic disks supported in a cluster?
· Can a cluster disk be extended without rebooting?
· How can I replace a disk that has gone bad in a cluster?
· Why is zoning needed to isolate a Server cluster in a SAN?
· Can a cluster server boot from a SAN?
· Is Kerberos authentication possible for services hosted on a cluster?
· Is IIS cluster-aware?
· Can Server clusters span multiple sites?
· Will Microsoft provide a cluster file system?
Introduction to Server Clusters
Q. What is a Server Cluster?
A. A Server cluster is a collection of independent servers that together provide a single, highly available platform for hosting applications
Q. What are the benefits of Server clusters?
A. There are three primary benefits to server clustering: availability, manageability, and scalability:
Availability:
Server clusters provide a highly available platform for deploying applications. A Server cluster ensures that applications continue to run in the event of planned downtime due to maintenance or unplanned downtime due to failures. Server clusters protect against hardware failures, failures of the Windows operating system, device drivers or application software. Server clusters allow operating system and application software to be upgraded across the cluster without having to take down the application.
Manageability:
Server clusters allow administrators to quickly inspect the status of all cluster resources and move workloads around onto different servers within the cluster. This is useful for manual load balancing, and to perform "rolling updates" on the servers without taking important data and applications offline.
Scalability:
Applications that can be partitioned can be spread across the servers of a cluster allowing additional CPU and memory to be applied to a problem. As the problem size increases, additional servers can be added to the cluster. A partitioned application is one where the data (or function) can be split up into independent units. For example, a customer database could be split into two units, one covering customers with names beginning A thru L and the other for customers with names beginning M thru Z.
Q. What are Server clusters used for?
A. Server clusters provide a highly available platform for mission critical, line of business applications. These typically include database servers (such as Microsoft SQL Server), mail and collaboration servers (such as Exchange Server) and infrastructure servers such as file servers and print servers.
Q. Is Server cluster the same as Microsoft Cluster Server (MSCS) and/or Wolfpack?
A. The code name for the original Server cluster project was Wolfpack. When the product was released as part of Microsoft Windows NT 4.0 Enterprise Edition it was named Microsoft Cluster Server or MSCS. In Microsoft Windows 2000, the official name of the product became Server cluster.
Q. How do Server clusters fit into the larger Windows high availability story?
A. Server clusters are part of a much larger story for highly available applications running on Microsoft products. Windows has two cluster technologies, server clusters and Network Load Balancing (NLB). Server clusters are used to ensure that stateful applications such as a database (e.g., SQL Server) can be kept highly available by failing over the application from one server to another in the event of a failure. NLB is used to distribute client requests across a set of identical servers. NLB is particularly useful for ensuring that stateless applications such as a web server (e.g., IIS) are highly available and can be scaled-out by adding additional servers as the load increases.
In addition, Microsoft Application Center 2000 includes Component Load Balancing (CLB) CLB is a technology used to ensure COM+ applications are highly available and can scale-out.
The Windows platform also uses other technologies to ensure high availability and scalability. For example, Active Directory is made highly available through replication of the directory contents across a set of domain controllers.
Q. What is Scale-up verses Scale-out?
A. Scale-up is a term used to describe the scalability provided by large symmetric multi-processing (SMP) systems. As the load increases, more processors and more memory can be added to a single server, increasing the processing power and memory available for applications. Windows 2000 Datacenter Server and Windows Server 2003 Datacenter Edition provide the support for scaling up to large computers with many CPUs and large physical memory. In a scale-out environment, Server clusters are deployed to ensure that the failure of the server does not cause the applications or services to fail. Scale-up is typically done for either hosting large applications that cannot be partitioned, or to consolidate a set of applications into a single server (or small number of servers in the case of a server cluster).
Scale-out is a term used to describe the idea that an application can be scaled by partitioning the workload and spreading it across a set of cooperating servers. If the load increases, additional servers can be added to the set to provide additional processing power and memory.
Server clusters are particularly useful for enhancing the availability of applications that can be scaled out across a set of nodes by allowing the individual pieces or partitions of a scale-out solution to be made highly available via failover.
Q. Is Server cluster a standard feature of all Windows operating system products?
A. Server clusters are not available in all Windows operating system products. Server clusters are available in:
· Windows NT Server, Enterprise Edition
· Windows 2000 Advanced Server and Datacenter server
· Windows Server 2003 Enterprise Edition and Datacenter Edition
· Windows Server 2003, 64 bit Enterprise Edition and Datacenter Edition
Server clusters are also available as part of the Server Appliance Kit, available to OEMs for building embedded solutions based on the Windows operating system.
Q. How do I know if my Server cluster is a supported configuration?
A. All server clusters must be qualified to be supported by Microsoft. A qualified configuration has undergone extensive testing using a hardware compatibility test provided by Microsoft. All qualified solutions appear on the Microsoft Hardware Compatibility List (HCL) (http://go.microsoft.com/fwlink/?LinkID=67738). Only cluster solutions listed on the HCL are supported by Microsoft.
The complete cluster solution must be listed on the Cluster HCL list. The complete solution includes the servers, the storage adapters, the interconnect type, the storage controllers firmware and driver versions. All of the components must match exactly, including any software, driver or firmware versions for the solution to be qualified.
The HCL contains a set of qualified cluster components. A solution built from qualified components does NOT imply that the solution is qualified.
The cluster component lists have been a source of confusion in the past and we will be removing the cluster component lists (such as Cluster/RAID) from the HCL for Windows Server 2003.
Q. Where are supported configurations listed?
A. All qualified solutions appear on the Microsoft Hardware Compatibility list (HCL) (http://go.microsoft.com/fwlink/?LinkID=67738). Only cluster solutions listed on the HCL are supported by Microsoft.
Q. Where can I find more documentation on Server clusters?
A. Server clusters are extensively covered in the online help in Windows 2000 and Windows Server 2003. Additional information can be found on the Microsoft web site at:
· http://go.microsoft.com/fwlink/?LinkID=67742
· http://go.microsoft.com/fwlink/?LinkID=67740
· http://go.microsoft.com/fwlink/?LinkID=67741
Availability
Q. Can Server clusters provide zero downtime for applications?
A.
· No. Server clusters can dramatically reduce planned and unplanned downtime. However, even with Server clusters, a server could still experience downtime from the following events:
· Failover time: If a Server cluster recovers from a server or application failure, or if it is used to move applications from one server to another, the application(s) will be unavailable for a non-zero period of time (typically under a minute).
· Failures from which Server clusters can not recover: There are types of failure that Server clusters do not protect against, such as loss of a disk not protected by RAID, loss of power when a UPS is not used, or loss of a site when there is no fast-recovery disaster recovery plan. Most of these can be survived with minimal downtime if precautions are taken in advance.
· Server maintenance that requires downtime: Server clusters can keep applications and data online through many types of server maintenance, but not all (for example: installing a new version of an application which has a new on-disk data format that requires reformatting preexisting data).
Microsoft recommends that clusters be used as one element in customers' overall programs to provide high integrity and high availability for their mission-critical server-based data and applications.
Q. Which types of applications and services benefit from Server clustering?
A. There are three types of server applications that will benefit from Server clusters:
1. "In the box" services provided by the Windows platform: For example: File shares, print queues, Microsoft Message Queue Server (MSMQ) services, and Component Services (formerly known as Transaction Server) services.
2. Generic applications: Server clusters include a point-and-click wizard for setting up any well-behaved server application for basic error detection, automatic recovery, and operator-initiated management (e.g., move from one server to the other). A "well behaved" server application is one which keeps a recoverable state on cluster disk(s), and whose client can gracefully handle a pause in service as the application is automatically re-started.
3. Cluster-aware applications: Software vendors test and support their application products on Server clusters.
Manageability and Deployment
Q. Do Server clusters provide a single image view for administration?
A. Yes, Server clusters provide administrators with a graphical interface (Cluster Administrator) and a command line tool (Cluster.exe available on Windows 2000 and Windows Server 2003) from which they can monitor and manage all of the resources in a cluster as if it was a single system. The administrator can:
· monitor the status of all servers and applications in the cluster
· setup new applications, file shares, print queues, etc. for high availability
· administer the recovery policies for applications and resources
· take applications offline, bring them back online, and move them from one server to another
The ability to graphically move workload from one server to another with only a momentary pause in service (typically less than a minute), means administrators can easily unload servers for planned maintenance without taking important data and applications offline for long periods of time.
Q. Can Server clusters be managed remotely?
A. Yes, an authorized cluster administrator can run Cluster Administrator from any Windows NT or Windows 2000 Workstation or Server, any Windows XP machine or any Windows Server 2003 on the network. The cluster.exe command line tool can be run remotely from any Windows 2000 Workstation or Server, any Windows XP machine or any Windows Server 2003 on the network.
Q. Can Server clusters be setup and configured remotely?
A. Yes, with Windows Server 2003, Cluster administrator and the cluster.exe command line tool can also be used to remotely setup and configure a Server cluster (e.g., create a new cluster, add a new server to an existing cluster or remove servers from a cluster).
Q. Is the server cluster software installed by default?
A. On Windows NT and Windows 2000, the Server cluster software is not installed by default. On Windows NT you must install the Enterprise Edition CD. With Windows 2000, you must install the Server cluster software using the optional component management tool.
With Windows Server 2003, the Server cluster software is installed by default on Enterprise Edition and Datacenter Edition. The cluster software is NOT configured by default and will not start until the server is either used to create a new cluster or the server is added to an existing cluster. In Windows Server 2003, you must use Cluster Administrator or the Cluster.exe command line tool to configure a cluster.
Q. Can a Server cluster be installed and configured using unattended installs?
A. Yes, to perform an unattended installation for a Windows 2000 Cluster using an Uttend.txt file, please consult the Windows 2000 Deployment Guide located on the Windows 2000 CD-ROM in the following location: %CDROM%\SUPPORT\TOOLS\DEPLOY.CAB\Unattend.doc
Additional information can be found in article 176507 in the Microsoft Knowledge Base (http://go.microsoft.com/fwlink/?LinkID=67745).
Q. Do applications have to be installed separately on all the cluster servers?
A. It depends on the application. Each application that can be failed over between servers in a cluster must be available on all servers in the cluster. Historically, application setup has not been cluster-aware and therefore the application must be installed separately on each server.
With more recent versions of some applications, for example SQL Server 2000, the application setup is aware of Server clusters. In the case of Microsoft SQL Server 2000 setup, the appropriate files are copied to all servers and registry settings and other SQL Server configuration is done just once.
Q. How can I take advantage of extensibility features of ISA Server?
A. A number of third-party vendors offer solutions such as virus detection, content filtering, site categorization, reporting, and administration. Customers and developers also have the ability to create their own extensions to ISA Server. ISA Server includes a comprehensive software development kit for developing tools that build on ISA Server firewall, caching, and management features.
Q. Are cluster management operations scriptable?
A. Yes, there are a number of options for scripting management operations:
· The Cluster.exe command line tool allows command files to be built that can change the state of cluster resources, move resources around etc.. For more information about the command line, see the online help or type cluster at the command line on a server that has Server clusters installed or has the Windows server admin pack installed.
· In Windows 2000 and Windows Server 2003, there is a scriptable COM interface to the Server cluster APIs. This allows VBScript or any other scripting language supported by Windows to call the cluster APIs. These APIs provide a way to change the state of the cluster as well as return data about the resources or applications in a cluster. For more information about the cluster server APIs, see the Platform SDK, it has comprehensive documentation for the cluster APIs and the COM (automation server) APIs.
· In Windows Server 2003, there is a WMI provider for Server clusters that allows WMI scripts to manage the cluster. For more information about the cluster server WMI schema, see the Platform SDK.
Q. Can command line tools be used to manage a cluster?
A. Yes, Server clusters have a command line tool cluster.exe that can be used to manage the cluster. With Windows Server 2003, this command line tool can also be used to create new clusters, add servers to existing clusters and remove servers from clusters. Almost all of the functionality provided by Cluster Administrator is available through cluster.exe in Windows Server 2003.
Q. Is WMI supported for managing a cluster?
A. Yes, in Windows Server 2003, there is a WMI provider that allows a Server cluster to be managed. In addition, all the Server cluster events (such as server up and server down, resource online, offline, failed etc.) are available through WMI events.
Q. Does Server clusters support WMI eventing?
A. Yes, in Windows Server 2003, all of the Server cluster events (such as server up and server down, resource online, offline, failed etc.) are available through WMI events.
Q. Can Group Policy be used on cluster servers?
A. You can apply Group Policy to cluster servers. There are a few things to remember. Applications failover from one server to another in a cluster and will typically expect the same policies to be in effect no matter where they are hosted. You should ensure that all cluster servers have the same set of group policies applied to them.
With Windows Server 2003 and Windows 2000 SP3 onwards, virtual servers can be published in active directory. You should NOT apply group policies to a virtual server computer object. The policy will only be applied to the server that is currently hosting the virtual server and all policy settings may be lost if the application fails over.
Q. Is Systems Management Server (SMS) supported for deploying applications in a Server cluster?
A. No, at this time, SMS is not supported for deploying applications across a cluster.
Q. Is Application Center 2000 supported on a Server cluster?
A. No, Application Center 2000 provides a set of features that are intended for managing servers hosting web server front-end and middle-tier business logic. Server clusters are typically deployed to ensure high availability of the back-end database engine.
Q. Are MOM management packs available for monitoring Server clusters?
A. No, at this time, there are no server-aware MOM management packs.
Q. How do server clusters help administrators perform rolling upgrades?
A. With Server clusters, server administrators no longer have to do all of their maintenance within those rare windows of opportunity when no users are online. Instead, they can simply wait until a convenient off-peak time when one of the servers in the cluster can be removed for maintenance and the workload distributed across the other cluster servers. By pointing-and-clicking in Cluster Administrator (or using scripts) to move all of the workload onto other servers, one of the servers in the cluster can be upgraded. Once the upgrade is complete and tested, that server can be rebooted where it will automatically re-join the cluster, ready for work. When convenient, the administrator repeats the process to perform maintenance on the other servers in the cluster. This ability to keep applications and data online while performing server maintenance is often referred to as doing "rolling upgrades" to your servers.
Server clusters allows rolling upgrades for a new release and the previous version.
Q. What combination of Windows versions supports rolling upgrades?
A. Server clusters allow rolling upgrades for a new release and the previous version. The following rolling upgrades are supported:
· Windows NT Enterprise Edition to Windows 2000
· Windows 2000 to Windows Server 2003
Q. Can I manage different versions of Server cluster from a single Cluster Administrator tool?
A. Yes, the Server cluster tools allow a mixed version cluster to be managed from a single point. During a rolling upgrade, a cluster may contain different versions of the operating system. As an example Windows 2000 and Windows Server 2003 can co-exist in the same cluster. The administration tools provided with Windows Server 2003 allow such a mixed-version cluster to be managed.
Q. Are there other manageability benefits with Server clusters?
A. Server clusters allow administrators to quickly inspect the status of all cluster resources, and move workload around onto different servers within the cluster. This is useful for manual load balancing, and to perform "rolling updates" on the servers without taking important data and applications offline.
Scalability (Server Clusters: Frequently Asked Questions for Windows 2000 and Windows Server 2003)
Q. Do Server clusters enhance server scalability?
A. The primary goal of Server clusters is to provide a highly available platform for applications.
There are some types of applications which can be scaled-out to take advantage of a set of machines. More machines can be added to the set to increase the CPU or memory available to the application. Applications of this type are typically scaled by partitioning the data set. For example, a SQL Server database may be scaled out by partitioning the database into pieces and then using database views to give the client applications the illusion of a single database.
Server clusters do not provide facilities to partition or scale-out an application; however, Server clusters allow such applications to be scaled-out in a highly available environment. Each of the partitions can be failed over independently within the set of machines (or to additional spare nodes) so that in the event of failure, a partition of the application remains available.
Q. Do Server clusters provide load balancing of applications?
A. Server clusters provide a manual load balancing mechanism. Applications can be moved around the servers of a cluster to distribute the load.
There are no automated tools provided with the operating system to automatically load balance applications and no enhanced failover policies that use load to determine the best placement for an application.
Server cluster Concepts (Server Clusters: Frequently Asked Questions for Windows 2000 and Windows Server 2003)
Q. What hardware do you need to build a Server cluster?
A. The most important criteria for Server cluster hardware is that it be included in a validated Cluster configuration on the Microsoft Hardware Compatibility List (HCL), indicating it has passed the Microsoft Cluster Hardware Compatibility Test. All qualified solutions appear on the Microsoft HCL (http://go.microsoft.com/fwlink/?linkid=67738). Only cluster solutions listed on the HCL are supported by Microsoft.
In general, the criteria for building a server cluster include the following:
· Servers: Two or more PCI-based machines running one of the operating system releases that support Server clusters (see below). Server clusters can run on all hardware architectures supported by the base Windows operating system, however, you cannot mix 32-bit and 64-bit architectures in the same cluster.
· Storage: Each server needs to be attached to a shared, external storage bus(es) that is/are separate from the bus containing the system disk, the startup disk or the pagefile disk. Applications and data are stored on one or more disks attached to this bus. There must be enough storage capacity on the shared cluster bus(es) for all of the applications running in the cluster environment. This shared storage configuration allows applications to failover between servers in the cluster.
Microsoft recommends hardware Redundant Array of Inexpensive Disks (RAID) for all cluster disks to eliminate disk drives as a potential single point of failure. This means using a RAID storage unit, a host-based RAID adapter that implements RAID across disks, etc.
SCSI is supported for 2-node cluster configurations only. Fibre channel arbitrated loop is supported for 2-node clusters only. Microsoft recommends using fibre channel switched fabrics for clusters of more than two nodes.
· Network: Each server needs at least two network cards. Typically, one is the public network and the other is a private network between the two nodes. A static IP address is needed for each group of applications that move as a unit between nodes. Server clusters can project the identity of multiple servers from a single cluster by using multiple IP addresses and computer names: this is known as a virtual server.
Q. What is a cluster resource?
A. A cluster resource is the lowest level unit of management in a Server cluster. A resource represents a physical object or an instance of running code. For example, a physical disk, an IP address, an MSMQ queue, a COM object all of these things are considered to be resources. From a management perspective, resources can be independently started and stopped and each is monitored to ensure that it is healthy.
Server cluster can monitor any arbitrary resource type. This is possible because Server clusters define a resource plug-in model. Each resource type has an associated resource plug-in or resource dll that is used to start, stop and provide health information that is specific to the resource type. For example, starting and stopping SQL Server is different from starting and stopping a physical disk. The resource dll takes care of the differences. Application developers and system administrators can build new resource dlls for their applications that can be registered with the cluster service.
Server clusters provides some generic plug-ins that can be used to make existing applications cluster-aware very quickly, known as Generic Service and Generic Application. With Windows Server 2003, a Generic Script resource plug-in was added that allows the resource dll to be written in any scripting language supported by the Windows operating system.
Q. What is a resource dependency?
A. A complete application actually consists of multiple pieces or multiple resources, some pieces are code and others are physical resources required by the application. The resources are related in different ways; for example, an application that writes to a disk cannot come online until the disk is online. If the disk fails, then, by definition, the application cannot continue to run since it writes to the disk. Resource dependencies can be defined by the application developer or system administrator to capture these relationships. Resource dependencies define the order that resources are brought online and control how failures are propagated to the various pieces of the application.
Q. What is a resource group?
A. A resource group is a collection of one or more resources that are managed and monitored as a single unit. A resource group can be started or stopped. If a resource group is started, each resource in the group is started (taking into account any start order defined by the dependencies between resources in the group). If a resource group is stopped, all of the resources in the group are stopped. Dependencies between resources cannot span a group. In other words, the set of resources within a group is an autonomous unit that can be started and stopped independently from any other group. A group is a single, indivisible unit that is hosted on one server in a Server cluster at any point in time and it is the unit of failover.
Q. Can I have dependencies between resources in different groups?
A. No, resource dependencies are confined to a single group.
Q. What is a virtual server?
A. A virtual server is a resource group that contains an IP address resource and a network name resource. When an application is hosted in a virtual server, the application can be accessed by clients using the IP address or network name in that resource group. As the resource group fails over across the cluster, the IP address and network name remain the same, therefore the client becomes unaware of the physical location of the application and will continue to work in the event of a failure of one of the servers in the cluster.
Q. How can I take advantage of extensibility features of ISA Server?
A. A number of third-party vendors offer solutions such as virus detection, content filtering, site categorization, reporting, and administration. Customers and developers also have the ability to create their own extensions to ISA Server. ISA Server includes a comprehensive software development kit for developing tools that build on ISA Server firewall, caching, and management features.
Q. What is failover?
A. Server clusters monitor the health of the nodes in the cluster and the resources in the cluster. In the event of a server failure, the cluster software re-starts the failed server's workload on one or more of the remaining servers. If an individual resource or application fails (but the server does not), Server clusters will typically try to re-start the application on the same server; if that fails, it moves the application's resources and re-starts it on the other server. The process of detecting failures and restarting the application on another server in the cluster is known as failover.
The cluster administrator can set various recovery policies such as whether or not to re-start an application on the same server, and whether or not to automatically "failback" (re-balance) workloads when a failed server comes back online.
Q. Is failover transparent to users?
A. Server clusters do not require any special software on client computers, so the user experience during failover depends on the nature of the client side of their client-server application. Client reconnection can be made transparent, because the Server clusters software has restarted the applications, file shares, etc. at exactly the same IP address.
If a client is using "state-less" connections such as a standard browser connection, then the client would be unaware of a failover if it occurred between server requests. If a failure occurs while a client is connected to the failed resource, then the client will receive whatever standard notification is provided by the client side of their application when the server side becomes unavailable. This might be, for example, the standard "Abort, Retry, or Cancel?" prompt you get when using the Windows Explorer to download a file at the time a server or network goes down. In this case, client reconnection is not automatic (the user must choose "Retry"), but the user is fully informed of what is happening and has a simple, well-understood method of re-establishing contact with the server. Of course, in the meantime, the cluster service is busily re-starting the service or application so that, when the user chooses "Retry", it re-appears as if it never went away.
Q. What is failback?
A. In the event of the failure of a server in a cluster, the applications and resources are failed over to another node in the cluster. When the failed node rejoins the cluster (after reboot for example), that node now is free to be used by applications. A cluster administrator can set policies on resources and resource groups that allow an application to automatically move back to a node if it becomes available, thus automatically taking advantage of a node rejoining the cluster. These policies are known as failback policies. You should take care when defining automatic failback policies since depending on the application, automatically moving the application (which was working just fine) may have undesirable consequences on the clients using the applications.
Q. When an application restarts after failover, does it restore the application state at the time of failure?
A. No, Server clusters provide a fast crash restart mechanism. When an application is failed over and restarted, the application is restarted from scratch. Any persistent data written out to a database or to files is available to the application, but any in-memory state that the application had before the failover is lost.
Q. At what level does failover exist?
A. At the resource group level.
Q. What is a Quorum Resource and how does it help Server clusters provide high availability?
A. Server clusters require a quorum resource to function. The quorum resource, like any other resource, is a resource which can only be owned by one server at a time, and for which servers can negotiate for ownership. Negotiating for the quorum resource allows Server clusters to avoid "split-brain" situations where the servers are active and think the other servers are down. This can happen when, for example, the cluster interconnect is lost and network response time is problematic. The quorum resource is used to store the definitive copy of the cluster configuration so that regardless of any sequence of failures, the cluster configuration will always remain consistent.
Q. What is active/active verses active/passive?
A. Active/Active and Active/Passive are terms used to describe how applications are deployed in a cluster. Unfortunately, they mean different things to different people and so the terms tend to cause confusion.
From the perspective of a single application or database:
· Active/Active means that the same application or pieces of the same service can be run concurrently on different nodes in the cluster. For example SQL Server 2000 can be configured such that the database is partitioned and each node can be running a single instance of the database. SQL Server provides the notion of views to provide a single image of the entire database.
· Active/Passive means that only one node in the cluster can be hosting the given application. For example, a single file share is active/passive. Any given file share can only be hosted on one node at a time.
From the perspective of a set of instances of an application or service:
· Active/Active means that different instances of the same application can be running concurrently on different cluster nodes. For example, each node in a cluster can be running SQL Server against a different database. A single cluster can support many file shares that are hosted on the nodes in a cluster concurrently.
· Active/Passive means that only one instance of a service can be running anywhere in the cluster. For example, there must only be a single instance of the DHCP service running in the cluster at any point in time.
From the perspective of the cluster:
· Active/Active means that all nodes in the cluster are running applications. These may be multiple instances of the same application or different applications (for example, in a 2-node cluster, WINS may be running on one node and DHCP may be running on the other node).
· Active/Passive means that one of the cluster nodes is spare and not being used to host applications.
Server clusters support all of these different combinations; the terms are really about how specific applications or sets of applications are deployed.
With the advent of more than two servers in a cluster, starting with Windows 2000 Datacenter, the term active/active is confusing because there may be four servers. When there are multiple servers, the set of options available for deployment becomes more flexible, allowing different configurations such as N+I.
Q. How do I benefit from more than two nodes in a cluster?
A. Failover is the mechanism that instance applications and the individual partitions of a partitioned application typically employ for high availability (the term Pack has been coined to describe a highly available, single instance application or partition).
In a 2-node cluster, defining failover policies is trivial. If one node fails, the only option is to failover to the remaining node. As the size of a cluster increases, different failover policies are possible and each one has different characteristics.
Failover Pairs
In a large cluster, failover policies can be defined such that each application is set to failover between two nodes. The simple example below shows two applications App1 and App2 in a 4-node cluster.
Figure 1: Failover pairs
Configuration has pros and cons:
Pro
Good for clusters that are supporting heavy-weight1 applications such as databases. This configuration ensures that in the event of failure, two applications will not be hosted on the same node.
Pro
Very easy to plan capacity. Each node is sized based on the application that it will need to host (just like a 2-node cluster hosting one application).
Pro
Effect of a node failure on availability and performance of the system is very easy to determine.
Pro
Get the flexibility of a larger cluster. In the event that a node is taken out for maintenance, the buddy for a given application can be changed dynamically (may end up with standby policy below).
Con
In simple configurations such as the one above, only 50% of the capacity of the cluster is in use.
Con
Administrator intervention may be required in the event of multiple failures.
1 A heavy-weight application is one that consumes a significant number of system resources such as CPU, memory or IO bandwidth.
Failover pairs are supported by server clusters on all versions of Windows by limiting the possible owner list for each resource to a given pair of nodes.
Hot-Standby Server
To reduce the overhead of failover pairs, the spare node for each pair may be consolidated into a single node, providing a hot standby server that is capable of picking up the work in the event of a failure.
Figure 2: Standby Server
Configuration has pros and cons:
Pro
Good for clusters that are supporting heavy-weight applications such as databases. This configuration ensures that in the event of a single failure, two applications will not be hosted on the same node.
Pro
Very easy to plan capacity. Each node is sized based on the application that it will need to host, the spare is sized to be the maximum of the other nodes.
Pro
Effect of a node failure on availability and performance of the system is very easy to determine.
Con
Configuration is targeted towards a single point of failure.
Con
Does not really handle multiple failures well. This may be an issue during scheduled maintenance where the spare may be in use.
Server clusters support standby servers today using a combination of the possible owners list and the preferred owners list. The preferred node should be set to the node that the application will run on by default and the possible owners for a given resource should be set to the preferred node and the spare node.
N+I
Standby server works well for 4-node clusters in some configurations, however, its ability to handle multiple failures is limited. N+I configurations are an extension of the standby server concept where there are N nodes hosting applications and I nodes spare.
Figure 3: N+I Spare node configuration
Configuration has pros and cons:
Pro
Good for clusters that are supporting heavy-weight applications such as databases or Exchange. This configuration ensures that in the event of a failure, an application instance will failover to a spare node, not one that is already in use.
Pro
Very easy to plan capacity. Each node is sized based on the application that it will need to host.
Pro
Effect of a node failure on availability and performance of the system is very easy to determine.
Pro
Configuration works well for multiple failures.
Con
Does not really handle multiple applications running in the same cluster well. This policy is best suited to applications running on a dedicated cluster.
Server cluster supports N+I scenarios in the Windows Server 2003 release using a cluster group public property AntiAffinityClassNames. This property can contain an arbitrary string of characters. In the event of a failover, if a group being failed over has a non-empty string in the AntiAffinityClassNames property, the failover manager will check all other nodes. If there are any nodes in the possible owners list for the resource that are NOT hosting a group with the same value in AntiAffinityClassNames, then those nodes are considered a good target for failover. If all nodes in the cluster are hosting groups that contain the same value in the AntiAffinityClassNames property, then the preferred node list is used to select a failover target.
Failover Ring
Failover rings allow each node in the cluster to run an application instance. In the event of a failure, the application on the failed node is moved to the next node in sequence.
Figure 4: Failover Ring
Configuration has pros and cons:
Pro
Good for clusters that are supporting several small application instances where the capacity of any node is large enough to support several at the same time.
Pro
Effect on performance of a node failure is easy to predict.
Pro
Easy to plan capacity for a single failure.
Con
Configuration does not work well for all cases of multiple failures. If one Node 1 fails, Node 2 will host two application instances and Nodes 3 and 4 will host one application instance. If Node 2 then fails, Node 3 will be hosting three application instances and Node 4 will be hosting one instance
Con
Not well suited to heavy-weight applications since multiple instances may end up being hosted on the same node even if there are lightly-loaded nodes.
Failover rings are supported by server clusters on the Windows Server 2003 release. This is done by defining the order of failover for a given group using the preferred owner list. A node order should be chosen and then the preferred node list should be set up with each group starting at a different node.
Random
In large clusters or even 4-node clusters that are running several applications, defining specific failover targets or policies for each application instance can be extremely cumbersome and error prone. The best policy in some cases is to allow the target to be chosen at random, with a statistical probability that this will spread the load around the cluster in the event of a failure.
Configuration has pros and cons:
Pro
Good for clusters that are supporting several small application instances where the capacity of any node is large enough to support several at the same time.
Pro
Does not require an administrator to decide where any given application should failover to.
Pro
Provided that there are sufficient applications or the applications are partitioned finely enough, this provides a good mechanism to statistically load balance the applications across the cluster in the event of a failure.
Pro
Configuration works well for multiple failures.
Pro
Very well tuned to handling multiple applications or many instances of the same application running in the same cluster well.
Con
Can be difficult to plan capacity. There is no real guarantee that the load will be balanced across the cluster.
Con
Effect on performance of a node failure is not easy to predict.
Con
Not well suited to heavy-weight applications since multiple instances may end up being hosted on the same node even if there are lightly-loaded nodes.
The Windows Server 2003 release of server clusters randomizes the failover target in the event of node failure. Each resource group that has an empty preferred owners list will be failed over to a random node in the cluster in the event that the node currently hosting it fails.
Customized control
There are some cases where specific nodes may be preferred for a given application instance.
Configuration has pros and cons:
Pro
Administrator has full control over what happens when a failure occurs.
Pro
Capacity planning is easy, since failure scenarios are predictable.
Con
With many applications running in a cluster, defining a good policy for failures can be extremely complex.
Con
Very hard to plan for multiple cascaded failures.
Server clusters provide full control over the order of failover using the preferred node list feature. The full semantics of the preferred node list can be defined as:
Preferred Node List
Move group to best possible initiated via administrator
Failover due to node or group failure
Contains all nodes in cluster
Group is moved to highest node in preferred node list that is up and running in the cluster.
Group is moved to the next node on the preferred node list.
Contains a subset of the nodes in the cluster
Group is moved to highest node in preferred node list that is up and running in the cluster.
If no nodes in the preferred node list are up and running, the group is moved to a random node.
Group is moved to the next node on the preferred node list.
If the node that was hosting the group is the last on the list or was not in the preferred node list, the group is moved to a random node.
Empty
Group is moved to a random node.
Group is moved to a random node.
Q. How many resources can be hosted in a cluster?
A. The theoretical limit for the number of resources in a cluster is 1,674; however, you should be aware that the cluster service periodically polls the resources to ensure they are alive. As the number of resources increases, the overhead of this polling also increases.
Server Requirements
Q. How many servers can be clustered?
A. The number of servers in a Server cluster is dependent on the Windows product and the Windows release. The following table lists the server sizes:
Windows Operating System
Maximum number of servers
Windows NT Enterprise Edition
2
Windows 2000 Advanced Server
2
Windows 2000 Datacenter Server
4
Windows Server 2003 Enterprise Edition
8
Windows Server 2003 Datacenter Edition
8
Q. Is it necessary that the servers in a Server cluster be identical?
A. The cluster hardware compatibility test does not require that all the servers in a qualified cluster be identical. As cluster sizes increase and therefore the investment in hardware increases, it is likely that different types of servers will appear in a single cluster.
The qualification process and the listing process are being improved to allow heterogeneous solutions to be more easily defined and qualified. This is particularly important to OEMs where server families change relatively often and therefore the additional qualification passes required increases dramatically with the current process. The server itself has never been an issue during qualification, it is typically the HBA or other piece of the storage subsystem, and therefore there is no real reason to mandate exact servers in a given qualified solution.
Q. Can 32-bit servers and 64-bit servers be mixed in the same Server cluster?
A. No, a single Server cluster must contain all 32-bit servers or all 64-bit servers.
Q. Can I use any of my servers to make a Server cluster?
A. All qualified solutions appear on the Microsoft Hardware Compatibility list (HCL) (http://go.microsoft.com/fwlink/?linkid=67738). Only cluster solutions listed on the HCL are supported by Microsoft. You can use any servers that are listed as part of a configuration to build a complete solution.
Interconnects
Q. What types of networks are required for cluster interconnects?
A. The network interface controllers along with any other components used in certified cluster configurations must have the Windows logo and appear on the Microsoft Hardware Compatibility List. The interconnect itself must support TCP/IP and UDP traffic and must appear as a single, non-routed LAN segment or subnet.
Q. Do Server clusters support high-bandwidth, low latency interconnects through WinSock direct?
A. The Server cluster code itself does not use WinSock direct path for intra-cluster communication. All communication between cluster nodes is over TCP/IP or UDP. Server clusters can use, but may not take advantage of any high-bandwidth, low latency connection between nodes that looks like a standard NDIS network adapter.
Q. How many network adapters should a cluster server have?
A. The cluster nodes must be connected by two or more independent networks in order to avoid a single point of failure. The use of two local area networks (LANs) is required. Cluster configurations that have only one network are not supported. At least 2 networks must be configured for cluster communications. Typically, one network is a private network, configured for cluster communications only and the other is a public network, configured for both client access and cluster communications.
Q. Can a cluster support multiple adapters configured as public?
A. Yes, however, there are a couple of caveats:
· Each network adapter must be on a different subnet.
· You can create different IP address resources that can be associated with different adapters; however, there is no way to make a resource or application dependent on the two networks so that if one fails, the application will continue to be available. In Server clusters, the dependencies must all be online for the dependent application to be online.
For future versions of the cluster technology, we are working on a feature to allow more flexible dependencies such as: a resource can depend on A OR B for it to be online.
Q. Can a cluster support multiple private networks?
A. Yes.
Q. Why do I see heartbeat packets on the networks marked as client-only?
A. In Windows 2000 and beyond, the Server cluster software heartbeats through the public network as well as the private network regardless of the configuration of the public network. This is to ensure that the cluster service can detect failures of the public network adapters and can failover an application if the node currently hosting it cannot communicate with the outside world.
Q. How should the different networks be configured?
A. The cluster nodes must be connected by two or more independent networks in order to avoid a single point of failure. The use of two local area networks (LANs) is typical. The configuration of a cluster with nodes connected by only one network is not supported.
You should configure the private network as Internal Cluster Communication Only and the public network as All Communications.
Q. What type of information is carried over the cluster network?
A. The cluster service uses the cluster network for the following intra-cluster traffic:
· Server cluster query/management and control information
· Heartbeats for failure detection
· Intra-cluster communication to ensure tight consistency of cluster configuration
· Cluster join requests when a node reboots or recovers from a failure
Q. What types of protocol are supported for applications in a Server cluster?
A. Cluster-aware applications must use protocols based on top of TCP/IP. The cluster software only supports the TCP/IP protocols for failover out-of -the-box.
Q. Will failover occur if the public network on one server fails?
A. Yes, in Windows 2000 and above, additional heartbeats are sent across the public networks to detect public network and/or network adapter failures and to cause applications to failover if the node they are currently hosted on cannot communicate with the other nodes.
Q. Is network adapter teaming supported in a Server cluster?
A. Yes, however there are caveats. The use of network adapter teaming on all cluster networks concurrently is not supported. At least one of the cluster networks that are enabled for internal communication between cluster nodes must not be teamed. Typically, the un-teamed network is a private interconnect dedicated to this type of communication. The use of network adapter teaming on other cluster networks is acceptable; however, if communication problems occur on a teamed network, Microsoft Product Support Services may require that teaming be disabled. If this action resolves the problem or issue, then you must seek further assistance from the manufacturer of the teaming solution.
Q. Is DHCP supported for Server cluster virtual servers?
A. No, virtual servers must have static IP addresses.
Q. Is DHCP supported for Server cluster nodes?
A. Yes, addresses may be assigned to the physical nodes dynamically by DHCP, but manual configuration with static addresses is recommended.
Q. Is NetBIOS required to run Server clusters?
A. In Windows NT and Windows 2000, NetBIOS is required for Server clusters to function correctly.
In Windows Server 2003, the cluster service does not require NetBIOS. A basic principle of server security is to disable unneeded services. To determine whether to disable NetBIOS, consider the following:
· Some services and applications other than the cluster service use NetBIOS. Review all the ways a clustered server functions before disabling NetBIOS on it.
· With NetBIOS disabled, you will not be able to use the Browse function in Cluster Administrator when opening a connection to a cluster. Cluster Administrator uses NetBIOS to enumerate all clusters in a domain.
· With NetBIOS disabled, Cluster Administrator does not work if a cluster name is specified.
You can control the NetBIOS setting for the cluster through the Cluster IP Address resource property sheet (parameters).
Q. Is IPSec supported in a Server cluster?
A. Although it is possible to use Internet Protocol Security (IPSec) for applications that can failover in a server cluster, IPSec was not designed for failover situations and we recommend that you do NOT use IPSec for applications in a server cluster.
Q. How do Server clusters update router tables when doing IP failover?
A. As part of its automatic recovery procedures, the cluster service will issue IETF standard ARP "flush" commands to routers to flush the machine addresses related to IP addresses that are being moved to a different server.
Q. How does the Address Resolution Protocol (ARP) cause systems on a LAN to update their tables that translate IP addresses to physical machine (MAC) addresses?
A. The ARP specification states that all systems receiving an ARP request must update their physical address mapping for the source of the request. The source IP address and physical network address are contained in the request. As part of the IP address registration process, the Windows TCP/IP driver broadcasts an ARP request on the appropriate LAN several times. This request asks the owner of the specified IP address to respond with its physical network address. By issuing a request for the IP address being registered, Windows can detect IP address conflicts; if a response is received, the address cannot be safely used. When it issues this request, though, Windows specifies the IP address being registered as the source of the request. Thus, all systems on the network will update their ARP cache entries for the specified address, and the registering system becomes the new owner of the address.
Note that if an address conflict does occur, the responding system can send out another ARP request for the same address, forcing the other systems on the subnet to update their caches again. Windows does this when it detects a conflict with an address that it has successfully registered.
Q. Server clusters use ARP broadcasts to re-set MAC addresses, but ARP broadcasts don't pass routers. So what about clients behind the routers?
A. If the clients were behind routers, they would be using the router(s) to access the subnet where the cluster servers were located. Accordingly, the clients would use their router (gateway) to pass the packets to the routers through whatever route (OSPF, RIP, etc) is designated. The end result is that their packet is forwarded to a router on the same subnet as the cluster. This router's ARP cache is consistent with the MAC address(es) that have been modified during a failover. Packets thereby get to the correct Virtual server, without the remote clients ever having seen the original ARP broadcast.
Storage (Server Clusters: Frequently Asked Questions for Windows 2000 and Windows Server 2003)
There are many storage questions, these questions are categorized as general questions, questions about deploying Server clusters on a storage area network (SAN) and network attached storage (NAS) questions.
General Storage Questions
Q. What storage interconnects are supported by Server clusters?
A. Cluster Server does not limit the type of storage interconnects supported; however, there are some requirements of the storage subsystem that limit the types from a practical perspective. For example, all of the cluster nodes should be able to access the storage device. This typically impacts the interconnect since only interconnects that allow multiple initiators (i.e. nodes) can be used. The set of interconnects that are currently part of qualified configurations on the HCL include: SCSI (of various flavors), Fibre channel arbitrated loop and fibre channel switched fabrics.
Remember that only clusters where the full configuration appears on the Cluster HCL are supported by Microsoft.
Q. How do I configure the nodes and the storage on a SCSI cluster?
A. You must make sure that all of the devices on the SCSI bus have different SCSI Ids. By default, SCSI adapters tend to have Id 7. You should make sure that the adapters in each node have different Ids. Likewise, the disks should be given unique SCSI Ids on the bus.
For a SCSI bus to work correctly it must be terminated. There are many ways to terminate the bus, both internally (at the host adapter) and externally (using Y cables). To ensure that the cluster can survive different types of failures (specifically being able to power down one of the nodes), the SCSI bus should be terminated using passive components such as a Y cable. Internal termination, which requires the adapter to be powered up, is not recommended.
Note
Microsoft only allows 2-node clusters to be built using SCSI storage interconnects.
Q. Does Server clustering support fibre channel arbitrated loop (FC-AL)?
A. Yes, Microsoft only allows 2-node clusters to be built using FC-AL storage interconnects. Multiple clusters on a single fibre channel loop are NOT supported.
Q. Can multiple clusters be connected to the same storage controller?
A. Yes, there is a special device qualification test for storage controllers that ensures they respond correctly if multiple clusters are attached to the same controller. For multiple clusters to be connected to the same controller the storage controller MUST appear on the Multi-cluster device Hardware Compatibility List (HCL) AND each of the individual end-to-end cluster solutions must appear on the Cluster Hardware Compatibility List. For example: EMC Symmetrix 5.0 is on the multi-cluster device HCL list. Multiple clusters (say a Dell PowerEdge cluster and a Compaq Proliant cluster) can be connected to the EMC Symmetrix storage controller as long as Dell PowerEdge + EMC Symmetrix AND Compaq Proliant + EMC Symmetrix as both on the cluster HCL.
Q. Will failover occur if the storage cable is pulled from the host bus adapter (HBA)?
A. If the storage cable is pulled from the host bus adapter (HBA), there may be a pause before the adapter reacts to losing the connection, however, once the HBA has detected the communication failure, the disk resources within the cluster using the specific HBA will fail. This will trigger a failover to occur and the resource will be brought back on line on another node in the cluster.
If the storage cable is reconnected, the Windows operating system may not rescan for new hardware automatically (this depends on the driver used for the adapter). You may need to manually initiate a rescan for new devices. Once the rescan is done, the node can host any of the physical disk resources. If failback policies are set, any resources that failed over when the cable was removed will failback to the node when the cable is replaced.
Note
An HBA) is the storage interface that is deployed in the server. Typically this is a PCI card that connects the server to the storage fabric.
Q. Will Server clusters protect my disks from hardware failures?
A. No, Cluster server protects against server failure, operating system or application failure and planned downtime due to maintenance. Microsoft strongly suggests that application and user data is protected against disk failure using redundancy techniques such as mirroring, RAID or replication either in hardware or in software.
Q. Do Server clusters support RAID or mirrored disks?
A. Yes, Microsoft strongly suggests that application and user data is protected against disk failure using redundancy techniques such as mirroring, RAID or replication either in hardware or in software.
Q. Are dynamic disks supported in a cluster?
A. Windows server products shipped from Microsoft do not provide support for dynamic disks in a server cluster environment. The Volume Manager for Windows 2000 add-on product from Veritas can be used to add the dynamic disk features to a server cluster. When the Veritas Volume Manager product is installed on a cluster, Veritas should be the first point of support for cluster issues.
Q. Can a cluster disk be extended without rebooting?
A. Yes, cluster disks can be extended without rebooting if the storage controller supports dynamic expansion of the underlying physical disk. Many new storage controllers virtualize the Logical Units (LUNs) presented to the operating system and these controllers allow LUNs to be grown online from the storage controller management console. Microsoft provides a tool called DiskPart that allows volumes or partitions to be grown online to take advantage of the newly created space on the disk without disruption to applications or users using the disk. There are separate versions of DiskPart for Windows 2000 and Windows Server 2003. The Windows 2000 version is available as a free download on the web and the Windows Server 2003 version is shipped on the distribution media.
Note
An LUN equates to a disk device visible in Disk Administrator.
Q. Can additional disks be added to a cluster without rebooting?
A. Yes, you can insert a new disk or create a new LUN and make that visible to the cluster nodes. You should only make the disk visible to one node in the cluster and then create a cluster resource to protect that disk. Once the disk is protected, you can make the LUN visible to other nodes in the cluster. In some cases you may need to do a rescan in device manager to find the new disk. In other cases (especially with fibre channel) the disk may be automatically detected.
Q. Can disks be removed from the cluster without rebooting?
A. Yes
Q. What types of disks can be used for cluster disks?
A. Microsoft recommends that all partitions on the cluster disks be formatted with the NTFS file system. This is for two reasons. Firstly, NTFS provides access controls that can be used to secure the data on the disk. Secondly, NTFS can recover from volumes that have been forcibly dismounted; other file systems may become corrupt if they are forcibly dismounted.
Server clusters only support Master Boot Record (MBR) format disks. Cluster disks cannot be GPT format.
Q. Can tape devices or other non-disk devices be attached to the same storage bus as the Cluster Disks?
A. It depends on the storage interconnect. Server clusters use SCSI reserve and reset to arbitrate for cluster disks. In Windows NT and Windows 2000 cluster server performs an untargeted bus reset. In Windows Server 2003 it is possible to target the reset, however, it may fallback to an untargeted reset. If a tape device receives a reset, this will typically trigger the tape to rewind.
Server cluster does not provide any arbitration mechanism for tape devices that are visible from multiple servers, therefore tape devices are not protected against concurrent access from multiple servers.
Microsoft does NOT support attaching a tape device to the SCSI bus containing the cluster disks in the case of a SCSI cluster or the loop in and fibre channel arbitrated loop.
Tape devices can be attached to a switched fabric as long as they are not visible through the same adapters as the cluster disks. This can be achieved by either putting the tape drive in a different zone to the cluster disks or by LUN masking techniques.
Q. Are software fault tolerant disks (software RAID or mirroring) supported in a Server cluster?
A. Windows server products shipped from Microsoft do not provide support for software RAID or mirroring, however, there are 3rd party products that provide this functionality in a clustered environment.
Q. Is the Virtual Snapshot Service (VSS) supported in a Server cluster?
A. Yes, the Virtual Snapshot Service is new with Windows Server 2003 and provides basic snapshot capabilities that are used by backup applications to create consistent, single point in time backups. The cluster service has a VSS provider that allows the cluster service configuration to be snapshoted and stored as part of the system state by these backup applications.
Q. Do Timewarp snapshots work in a Server cluster?
A. No, Timewarp is a new feature in Windows Server 2003 that allows persistent snapshot to be created and exposed to clients. TImewarp makes use of features that are not cluster-aware and it is not supported in a cluster at this time.
Q. Are hardware snapshots or business recovery volumes supported in a Server cluster?
A. Yes, you can use facilities in the latest storage controllers to create snapshots of existing volumes. Note, however, when you create a snapshot of a disk you should NOT expose the snapshot back to the same cluster as the original disk. The cluster service uses the disk signature to uniquely identify a disk. With a snapshot, the disk and the snapshot have the same disk signature.
If you create a hardware snapshot or a business recovery volume of a cluster disk you should expose the snapshot to another server or cluster (typically a dedicated backup server).
Q. What other considerations are there when creating clustered disks?
A. Modern storage controllers provide a virtual view of the storage itself. A physical RAID set can be carved into multiple logical units that are exposed to the operating system as individual disks or LUNs. If you intend to carve up physical disks in this way and expose them as independent LUNs to the hosts, you should think carefully about the IO characteristics and the failure characteristics remember underneath, there is only a finite bandwidth to each spindle.
Microsoft recommends that you do not create a LUN for use as the quorum disk from the same underlying physical disks that you will be using for applications. The availability of the cluster is directly related to the availability of the quorum disk. If I/Os to the quorum disk take too long, the cluster server will assume that the quorum disk has failed and initiate a failover of the quorum device. At that point, all other cluster related activity is suspended until the quorum device is brought back online.
Q. How can I replace a disk that has gone bad in a cluster?
A. The answer depends on the Windows release:
· Windows NT Enterprise Edition
· FTEdit tool along with some manipulate of the registry. This is covered in article 243195 in the Microsoft Knowledge Base (http://go.microsoft.com/fwlink/?linkid=67761).
· Windows 2000
· DumpCfg.
· ClusterRecovery Reskit tool provided on Windows Server 2003 Reskit
· Windows Server 2003
· Automated system recovery.
· ConfDisk. This is covered in article 280425 in the Microsoft Knowledge Base (http://go.microsoft.com/fwlink/?linkid=67762).
· ClusterRecovery Reskit tool provided on Windows Server 2003 Reskit
Storage Area Networks (SAN) questions
Q. What is a Storage Area Network (SAN)?
A. A storage area network (SAN) is defined as a set of interconnected devices (e.g. disks and tapes) and servers that are connected to a common communication and data transfer infrastructure such as a fibre channel. The common communication and data transfer mechanism for a given deployment is commonly known as the storage fabric. The purpose of the SAN is to allow multiple servers access to a pool of storage in which any server can potentially access any storage unit. Clearly in this environment, management plays a large role providing security guarantees (who is authorized to access which devices) and sequencing or serialization guarantees (who can access which devices at what point in time).
Q. Why use a SAN?
A. Storage area networks provide a broad range of advantages over locally connected devices. They allow compute units to be detached from storage units, thereby allowing flexible deployment and re-purposing of both servers and storage dynamically to suit the current business needs without having to be concerned about buying the right devices for a given server or without re-cabling a datacenter to attach storage to a given server.
Q. Can a cluster be connected to a SAN?
A. Yes, Microsoft fully supports storage area networks both as part of the base Windows platform and as part of a complete Windows Clustering high availability solution.
Q. Can a cluster co-exist on a SAN with other servers?
A. One or more Server clusters can be deployed in a single SAN environment along with standalone Windows servers and/or with other non-Windows platforms.
Q. What additional SAN configuration is required to put a cluster on a shared SAN?
A.
· The cluster disks for each cluster on a SAN MUST be deployed in their own zone. All host bus adapters (HBAs) in a single cluster must be the same type and at the same firmware revision level. Many storage and switch vendors require that ALL HBAs on the same zone, and in some cases the same fabric, are the same type and have the same firmware revision number.
· All storage device drivers and HBA device drivers in a cluster must be at the same software version.
· When adding a new server to a SAN, ensure that the HBA is appropriate for the topology. In some configurations, adding an arbitrated loop HBA to a switched fibre fabric can result in widespread failures of the storage fabric. There have been real-world examples of this causing serious downtime.
Note
An HBA is the storage interface that is deployed in the server. Typically this is a PCI card that connects the server to the storage fabric.
Q. What is LUN masking or selective presentation?
A. LUN masking (also implemented as selective presentations) allows users to express at the controller level a specific relationship between a LUN and a host. Only the hosts that are configured to access the LUN should be able to see it.
Q. What is hardware zoning verses software zoning?
A. Zoning can be implemented in hardware/firmware on controllers or on software on the hosts. Microsoft recommends that controller based (or hardware) zoning be used since this allows for uniform implementation of access policy that cannot be interrupted or compromised by node disruption or failure of the software component.
Q. Why is zoning needed to isolate a Server cluster in a SAN?
A. The cluster uses mechanisms to protect access to the disks that can have an adverse effect on other clusters that are in the same zone. By using zoning to separate the cluster traffic from other cluster or non-cluster traffic, there is no chance of interference.
The diagram shows two clusters sharing a single storage controller. Each cluster is in its own zone. The LUNs presented by the storage controller must be allocated to individual clusters using fine-grained security provided by the storage controller itself. LUNs must be setup so that every LUN for a specific cluster is visible and accessible from all nodes of the cluster. A LUN should only be visible to one cluster at a time. The cluster software itself takes care of ensuring that although LUNs are visible to all cluster nodes, only one node in the cluster accesses and mounts the disk at any point in time.
The multi-cluster device test used to qualify storage configurations for the multi-cluster HCL list tests the isolation guarantees when multiple clusters are connected to a single storage controller in this way.
Q. Can a cluster server boot from a SAN?
A. Yes, however, there is a set of configuration restrictions around how Windows boots from a storage area network. For more information, see article 305547 in the Microsoft Knowledge Base (http://go.microsoft.com/fwlink/?linkid=67837).
Server clusters require that the startup disk, page file disk and system disk be on a different storage bus to the cluster server disks. To boot from a SAN, you must have a separate HBA for the boot, system and pagefile disks than the cluster disks. You MUST ensure that the cluster disks are isolated from the boot, system and pagefile disks by zoning the cluster disks into their own zone.
Q. Can I use multiple paths to SAN storage for high availability?
A. Microsoft does not provide a generic driver that allows multiple paths to the storage infrastructure for high availability; however, several vendors have built their own proprietary drivers that allow multiple HBAs and SAN fabrics to be used as a highly available storage infrastructure. For a Server cluster that has multi-path drivers to be considered supported, the multipath driver MUST appear as part of the complete cluster solution on the Cluster Hardware Compatibility List (HCL). NOTE: The driver version is VERY important and it MUST match the qualified version on the HCL.
Q. Can the startup disk, pagefile disks and the cluster disks be on the same SAN fabric?
A. No, in Windows Server 2003, there is a registry key that allows the startup disk, pagefile disks, and cluster disks to be on the same bus. This feature is enabled by a registry key, which helps ensure that it is not accidentally enabled by customers who do not understand the implications of this configuration. It is intended for OEMs to ship qualified and tested configurations and not for a typical end-user or administrator to set up in an ad hoc manner.
In the original release of Windows Server 2003, the registry key is:
HKLM\SYSTEM\CurrentControlSet\Services\ClusSvc\Parameters\ManageDisksOnSystemBuses 0x01
In Windows Server 2003 with Service Pack 1 (SP1), the registry key is:
HKLM\SYSTEM\CurrentControlSet\Services\ClusDisk\Parameters\ManageDisksOnSystemBuses 0x01
In Windows Server 2003 SP1, the key path was changed to use “Clusdisk” as a subkey instead of “ClusSvc.” This change was made to avoid issues during setup. However, the change is backward compatible, and systems that use the old key locations do not need to be modified.
Q. Can serverless backup be performed against cluster disks?
A. No, the cluster disk arbitration mechanism uses SCSI reserve and release operations. Once a server arbitrates for a cluster disk, that disk cannot be accessed by any other server on the storage network.
Network Attached Storage (NAS) questions
Q. What is Network Attached Storage (NAS)?
A. Network attached storage (NAS) is an alternative way to connect storage to servers that is built using standard network components such as Ethernet or other LAN technologies. The application servers access storage using file system functions such as open file, read file, write file, close file, etc.. These higher-level functions are encapsulated in protocols such as CIFS, NFS or AppleShare and run across standard IP-based connections.
Q. Can Server clusters use NAS for the shared storage?
A. Yes, providing the applications can store data on file shares and the file shares are accessible to the applications as they failover across the cluster, there is no reason why NAS cannot be used as the storage solution in a cluster.
There is currently no support in Windows to use NAS as the quorum resource.
In Windows Server 2003, we are providing a new quorum resource Majority Node Set that can be used to remove the need for a shared disk for the quorum resource. If you combine NAS storage with Majority Node Set quorum, you can build a failover cluster that does not require shared disks in the traditional sense of SCSI or SAN.
Highly Available File Servers
Q. Can I have an active/active file server?
A. The Windows 2000 resource kit contains a tool ClusTool that can be used to migrate file share settings from a single node to a cluster environment.
Q. Can FRS be used to replicate clustered file shares?
A. No, the file replication service (FRS) provided by Windows cannot be used to replicate a clustered file share. This means that clustered file shares cannot be the source or target for redundant links in a DFS tree. See the online documentation for DFS for more details.
Q. What file system types are supported in a cluster?
A. All partitions on clustered disks should be formatted with NTFS.
Q. Does client-side caching (offline folders) work with Server clusters?
A. Yes, in Windows Server 2003, you can select client-side caching (also know as offline folders) for clustered file shares.
Q. Is the Encrypting File System (EFS) supported on cluster disks?
A. With Windows Server 2003, the encrypting file system (EFS) is supported on clustered file shares. To enable EFS on a clustered file share, you must perform a number of tasks to configure the environment correctly:
· EFS can only be enabled on file shares when the virtual server has Kerberos enabled. By default, Kerberos is not enabled on a virtual server. To enable Kerberos you must check the Enable Kerberos Authentication check box on the network name resource that will be used to connect to the clustered file share. NOTE: Enabling Kerberos on a network name has a number of implications that you should ensure you fully understand before checking the box.
· All cluster node computer accounts, as well as the virtual server computer account, must be trusted for delegation. See online help for how to do this.
· To ensure that the users private keys are available to all nodes in the cluster, you must enable roaming profiles for users who want to store data using EFS. See online help for how to enable roaming profiles.
Once the cluster file shares have been created and the configuration steps above carried out, users data can be stored in encrypted files for added security.
Q. How many file shares can be hosted on a cluster?
A. The number of file shares in a cluster depends on the number of nodes in the cluster and the failure scenarios that you are trying to protect against. A single server has a limit for the number of file shares it can support so you need to take that into account when planning your cluster.
In a 2-node cluster, if one node fails, the remaining node must pick up all of the file shares. Therefore, to ensure the highest availability, the cluster should host the maximum number of shares that can be hosted by a single node.
Note
2-node Server clusters are focused on high availability, not scale-out, therefore you should not expect to hold more shares on a 2-node cluster than a single node.
In a 4-node cluster, you have other options that may be more appropriate, depending on the failure scenarios that you wish to protect against. For example, if you wish to survive one node failing at any point in time, you can configure the shares so that if one node fails, its work is spread across the remaining three nodes. This means that each node could be loaded to 66% of the maximum number of shares and still be within the maximum limit of a single node in the event of a single failure. In this case, the cluster can host three times the number of shares that a single server can host. If you wish to survive two nodes failing, then a 4-node cluster can hold twice as many shares (since if two nodes fail, the remaining two nodes need to pick up the load from the two failed servers) and so on.
In general, as the number of nodes in a cluster increases, the more options you have and the more you can use server clusters to scale-out a highly available infrastructure.
Q. What is the maximum capacity of a cluster disk?
A. Server cluster does not impose any restrictions on the size of a volume supported.
Q. How many disks can Server cluster support?
A. In Windows 2000, each clustered disk had to have a drive letter assigned to it, therefore the maximum number of clustered disks in a single cluster was limited to 23 volumes (26 letters of the alphabet minus A and B [Floppy drives] and C [system/boot drive]).
In Windows Server 2003, there is no longer a requirement for a clustered disk to have a driver letter assigned, therefore the number of disks is limited by the number that can be physically attached and the number supported by the underlying operating system.
Note
Applications can access disks with no drive letters in one of two ways a) directly using the object name associated with the disk or more likely b) by using mount points to link multiple disks together that can be accessed using a single drive letter.
Q. How can a cluster support more disks than there are drive letters?
A. Using file system mount points. For more information about using mount points with clustered disks see the online help for Windows Server 2003.
Q. Why can I browse shares owned by different virtual servers?
A. File shares are not scoped by the virtual server name that is hosting them. If you use a browsing tool (e.g. the NET VIEW command) you will see all the shares that are currently hosted on the physical node.
Highly Available Print Servers
Q. How do I cluster a printer?
A. Printers can be clustered using the Print Spooler cluster resource. The Windows 2000 and the Windows Server 2003 online help both give specific examples and the steps necessary to create a highly available print server.
Q. Can I have active/active print servers?
A. Yes, it is possible to host multiple print spoolers on a single Server cluster. The spoolers can be failed over independently and can run concurrently on multiple nodes in the cluster.
Q. How do I migrate printer settings from a single server to a cluster?
A. Microsoft provides a tool (Print Migrator) as part of the ResKit that can be used to migrate printer settings from one node to another or from one node to a Server cluster.
Q. How many printer resources can be hosted on a cluster?
A. The number of printers is limited by the number of resources the cluster can support, however, as the number of printers increases, so will the time to failover.
Removing all Single Points of Failure
Q. What other services does the server cluster rely on?
A. The cluster service itself relies on being able to authenticate and sign communications traffic between the cluster nodes. It uses the domain infrastructure to authenticate using the cluster service account. In an environment with Server clusters installed, you must ensure that the domain infrastructure is highly available; any disruption to the infrastructure can result in the clusters becoming unavailable.
Q. What other services do I need to think about?
A. In order for applications to remain highly available in a clustered environment, any services that the application requires external to the cluster must also be highly available. Many of these services have mechanisms such as replication or being made cluster-aware themselves to protect against failures. Examples of services that you should think about include WINS, DNS, DHCP, the domain infrastructure, firewalls, etc.
Q. What other single points of failure should I protect against?
A. Server clusters are a mechanism that protects applications against hardware, operating system and application failures. There are some types of hardware failure that you should think about:
· Disk failures you should use RAID or mirroring to protect against disk failures
· Hardware failures multiple hot swap fans in the server, redundant power supplies etc.
· Network failures redundant networks that do not have any shared components
· Site failures disaster recovery plans
Active Directory, DNS and Domain Controllers
Q. Is Kerberos authentication possible for services hosted on a cluster?
A. Yes, in Windows 2000 SP3 and above and Windows Server 2003, the cluster service publishes a computer object in Active Directory. This provides the infrastructure with sufficient state to allow Kerberos authentication against applications and services hosted in a virtual server.
For more information about Kerberos and how it works, see the TechNet web site (http://go.microsoft.com/fwlink/?linkid=67842).
Q. Can cluster servers also be domain controllers?
A. Yes, however, there are several caveats that you should fully understand before taking this approach. We recommend that Server cluster nodes are not domain controllers and that you co-locate a domain controller on the same subnet as the Server cluster public.
If you must make the cluster nodes into domain controllers, consider the following important points:
· If one cluster node in a 2-node cluster is a domain controller, all nodes must be domain controllers. It is recommended that you configure at least two of the nodes in a 4-node Datacenter cluster as domain controllers.
· There is overhead that is associated with the running of a domain controller. A domain controller that is idle can use anywhere between 130 to 140 megabytes (MB) of RAM, which includes the running of Windows Clustering. There is also replication traffic if these domain controllers have to replicate with other domain controllers within the domain and across domains. Most corporate deployments of clusters include nodes with gigabytes (GB) of memory so this is not generally an issue.
· If the cluster nodes are the only domain controllers, they each have to be DNS servers as well, and they should point to each other for primary DNS resolution, and to themselves for secondary DNS resolution. You have to address the problem of the ability to not register the private interface in DNS, especially if it is connected by way of a crossover cable (2-node only). For information about how to configure the heartbeat interface refer to article 258750 in the Microsoft Knowledge Base (http://go.microsoft.com/fwlink/?LinkID=46549). However, before you can accomplish step 12 in KB article 258750, you must first modify other configuration settings, which are outlined in article 275554 (http://go.microsoft.com/fwlink/?LinkID=67844).
If the cluster nodes are the only domain controllers, they must each be Global Catalog servers, or you must implement domainlets.
· The first domain controller in the forest takes on all flexible single master operation roles (refer to article 197132 at http://go.microsoft.com/fwlink/?LinkID=67847). You can redistribute these roles to each node. However, if a node fails over, the flexible single master operation roles that the node has taken on are no longer available. You can use Ntdsutil to forcibly take away the roles and assign them to the node that is still running (refer to article 223787 at http://go.microsoft.com/fwlink/?LinkID=67851). Review article 223346 at http://go.microsoft.com/fwlink/?LinkID=19807) for information about placement of flexible, single master operation roles throughout the domain.
· If a domain controller is so busy that the Cluster service is unable to gain access to the Quorum drive as needed, the Cluster service may interpret this as a resource failure and cause the cluster group to fail over to the other node. If the Quorum drive is in another group (although it should not be), and it is configured to affect the group, a failure may move all group resources to the other node, which may not be desirable. For more information regarding Quorum configuration, please refer to the article 280345 listed in the "Reference" section (http://go.microsoft.com/fwlink/?LinkID=67855).
· Clustering other programs such as SQL Server or Exchange Server in a scenario where the nodes are also domain controllers, may not result in optimal performance due to resource constraints. You should thoroughly test this configuration in a lab environment prior to deployment.
· You may want to consider making cluster nodes domain controllers (refer to KB article 171390 at http://go.microsoft.com/fwlink/?LinkID=67857 for more information), but if a domain controller is already local, or there is a reliable high-speed connectivity to a domain controller available, Microsoft does not recommend implementing them on cluster nodes.
Note
You must promote a cluster node to a domain controller by using the Dcpromo tool prior to installing Windows Clustering.
· You must be extremely careful when demoting a domain controller that is also a cluster node. When a node is demoted from a domain controller, the security settings and the user accounts are radically changed (user accounts are demoted to local accounts for example).
Q. Are virtual servers published in active directory?
A. Yes, in Windows 2000 SP3 and above and in Windows Server 2003, each virtual server has the option of being published in active directory.
Although the network name server cluster resource publishes a computer object in active directory, that computer object should NOT be used for administration tasks such as applying Group Policy. The ONLY role for the virtual server computer object in Windows 2000 and Windows Server 2003 is to allow Kerberos authentication and delegation and for cluster-aware, active directory-aware services (such as MSMQ) to publish service provider information.
Q. Is the cluster configuration stored in active directory?
A. No, at this time there is no cluster information other than the computer objects for virtual servers published in Active directory.
Q. Do Server clusters make domain controllers highly available?
A. No, domain controllers use replication across a set of servers to achieve high availability.
Q. How should my DNS server be configured to work with Server clusters?
A. The cluster service account needs to be able to publish records. In a secure, DNS backed zone, the DNS administrator can chose to restrict the access rights for users. The cluster service account must be granted permission to create records or alternatively, the records can be pre-created. If the records are pre-created, you should not set the zone to dynamic update.
Security Considerations (Server Clusters: Frequently Asked Questions for Windows 2000 and Windows Server 2003)
Q. How do I update the cluster service account password?
A. The cluster service account on ALL nodes in the cluster must match to ensure that the intra-cluster communication can be successfully authenticated. The cluster service itself sends messages between cluster nodes under a variety of conditions and if any of those communications fail, the cluster node will be removed from the cluster (i.e. the cluster service will be stopped). It is not possible to determine when the cluster service will establish communication and therefore there is no clear window that allows the cluster service account to be changed in a reliable way while ensuring that the cluster remains running.
Windows 2000
On Windows 2000, the cluster account password can only be reliably changed using the following steps:
1. Stop the cluster service on ALL nodes in the cluster
2. Change the password of the cluster service account at the domain controller
3. Update the service control manager password on ALL cluster nodes
4. Re-start the cluster service on all the cluster nodes
Windows Server 2003
The cluster.exe command on Windows Server 2003 has the ability to change the cluster account password dynamically without shutting down the cluster service on any of the nodes. The cluster.exe command changes the domain account password and updates the service control manager account information about all nodes in the cluster.
Cluster /cluster:cluster_name1[,cluster_name2,]/changepassword[:new_password[,old_password]] [/skipdc] [/force] [/options]
For more information refer to the online help for Windows Server 2003.
Q. What other security considerations and best practices do I need to worry about for Server clusters?
A. For security best practices, see the online help for Windows Server 2003.
In-the-box HA Services
Q. What highly available operating system services are provided on the Windows release?
A. Server clusters following highly available services by default in the Windows operating system:
· IP Address and Network Name: highly available network configuration to allow clients to location independent and failover unaware
· DHCP: Highly available DHCP server
· MSDTC: Highly available distributed transaction coordinator
· IIS: Highly available web server and FTP server*
· File Share: Highly available file share service and DFS
· Message Queue: Highly available MSMQ service
· MSMQ triggers: Highly available MSMQ trigger service (new for Windows Server 2003)
· Print Spooler: Highly available printer service
· WINS: Highly available WINS service
*In Windows Server 2003, IIS is made cluster-aware using the generic script resource and the scripts provided. There is no specific IIS resource type.
Q. Are MSMQ triggers supported in a Server cluster?
A. Yes, in Windows Server 2003, the MSMQ trigger service can be made highly available using Server clusters.
Q. Is IIS cluster-aware?
A. Yes, in Windows 2000, IIS web sites and FTP services can be made highly available using the IIS Server Instance resource type. In Windows Server 2003, the IIS Server Instance resource type was replaced with a set of generic scripts provided in the Windows Server 2003 release (see the online help for more information about converting IIS web servers and FTP servers from Windows 2000 to Windows Server 2003).
Although IIS web servers can be made highly available by failover using Server clustering, Microsoft recommends that you use a load balancing cluster such as provided by Network Load Balancing (NLB), another cluster mechanism provided by the Windows operating system to make IIS highly available and to scale-out a web service or web farm.
Depending on the access characteristics, you may choose either Server clusters or Network Load Balancing clusters to provide highly available FTP servers. Server clustering is good for FTP sites with high update rates or where you want to have a single copy of the FTP content. Network Load Balancing is good for mainly read-only FTP Servers.
Q. How is the IIS metabase kept consistent across the cluster?
A. The Windows operating system comes with a tool (IISSync) that allows the IIS metabase to be kept in sync across the nodes in the cluster. For more details see the online help.
Geographically Dispersed Clusters
Q. Can Server clusters span multiple sites?
A. Yes, Server clusters support a single cluster spanning multiple sites. This is known as a geographically dispersed cluster. All qualified geographically dispersed cluster solutions appear on the Microsoft Hardware Compatibility list (HCL) (http://go.microsoft.com/fwlink/?LinkID=67738). Only cluster solutions listed on the HCL are supported by Microsoft.
Q. How is a geographically dispersed cluster defined?
A. A geographically dispersed cluster is a Server cluster that has the following attributes:
1. Has multiple storage arrays, at least one deployed at each site. This ensures that in the event of failure of any one site, the other site(s) will have local copies of the data that they can use to continue to provide the services and applications.
2. Nodes are connected to storage in such a way that in the event of a failure of either a site or the communication links between sites, the nodes on a given site can access the storage on that site. In other words, in a two-site configuration, the nodes in site A are connected to the storage in site A directly and the nodes in site B are connected to the storage in site B directly. The nodes in site A con continue without accessing the storage on site B and vice-versa.
3. The storage fabric or host-based software provides a way to mirror or replicate data between the sites so that each site has a copy of the data. Different levels of consistency are available.
The following diagram shows a simple two-site cluster configuration.
Q. Will geographically dispersed clusters give me disaster tolerance, or disaster recovery?
A. The goal of multi-site Server cluster configurations is to ensure that loss of one site in the solution does not cause a loss of the complete application for business continuance and disaster recovery purposes. Sites are typically up to a few hundred miles apart so that they have completely different power, different communications infrastructure providers, and are placed so that natural disasters (e.g., earthquakes) are extremely unlikely to take out more than one site.
Geographically dispersed clusters do not provide disaster tolerance, since, in some cases, manual intervention is required to restart the applications.
Q. Can a geographically dispersed cluster use asynchronous replication between sites?
A. Yes, however, there are a couple of caveats:
· The quorum data must be synchronously replicated between the sites. To ensure that the Server cluster guarantees of consistency are met, the cluster database must be kept consistent across all nodes. If the quorum disk is replicated across the sites, it MUST be replicated synchronously.
In Windows Server 2003, a new quorum resource Majority Node Set can be used in these configurations as an alternative to replicating the quorum disk.
· If data is replicated asynchronously, in the event of a site failure, the secondary site will be consistent, but out of date. You should check that the applications can handle going back in time and that the client experience makes sense for the business. Applications such as SQL Server can be hosted on asynchronous replication; however, there are a bunch of restrictions and warning that you should be aware of (see the SQL Server high availability documentation for rules around multi-site replication of SQL Server data).
· If data is replicated at the block level, the replication must preserve the order of writes to the secondary site. Failure to ensure this will lead to data corruption.
Q. Does Microsoft provide a complete end-to-end geographically dispersed cluster solution?
A. No, Microsoft does not provide a software mechanism for replicating application data from one site to another in a geographically dispersed cluster. Microsoft works with hardware and software vendors to provide a complete solution. All qualified geographically dispersed cluster solutions appear on the Microsoft Hardware Compatibility list (HCL) (http://go.microsoft.com/fwlink/?LinkID=67738). Only cluster solutions listed on the HCL are supported by Microsoft.
Q. What additional requirements are there on a Server cluster to support multiple sites?
A. The Microsoft server clustering software itself is unaware of the extended nature of geographically dispersed clusters. There are no special features in Server cluster in Windows 2000 or Windows Server 2003 that are specific to these kinds of configuration. The network and storage architectures used to build geographically dispersed clusters must preserve the semantics that the server cluster technology expects. Fundamentally, the network and storage architecture of geographically dispersed server clusters must meet the following requirements:
1. The private and public network connections between cluster nodes must appear as a single, non-routed LAN (e.g., using technologies such as VLANs to ensure that all nodes in the cluster appear on the same IP subnets).
2. The network connections must be able to provide a maximum guaranteed round trip latency between nodes of no more than 500 milliseconds. The cluster uses heartbeat to detect whether a node is alive or not responding. These heartbeats are sent out on a periodic basis. If a node takes too long to respond to heartbeat packets, the cluster service starts a heavy-weight protocol to figure out which nodes are really still alive and which ones are dead; this is known as a cluster re-group. The heartbeat interval is not a configurable parameter for the cluster service (there are many reasons for this, but the bottom line is that changing this parameter can have a significant impact on the stability of the cluster and the failover time). 500ms round-trip is significantly below any threshold to ensure that artificial re-group operations are not triggered.
3. Windows 2000 requires that a cluster have a single shared disk known as the quorum disk. The storage infrastructure can provide mirroring across the sites to make a set of disks appear to the cluster service like a single disk, however, it must preserve the fundamental semantics that are required by the physical disk resource:
· Cluster service uses SCSI reserve commands and bus reset to arbitrate for and protect the shared disks. The semantics of these commands must be preserved across the sites even in the face of complete communication failures between sites. If a node on site A reserves a disk, nodes on site B should not be able to access the contents of the disk. These semantics are essential to avoid data corruption of cluster data and application data.
· The quorum disk must be replicated in real-time, synchronous mode across all sites. The different members of a mirrored quorum disk MUST contain the same data.
In Windows Server 2003, you can use either a mirrored/replicated quorum disk or a new resource Majority Node Set for a multi-site cluster.
Q. What additional requirements are there on applications?
A. As with the Server cluster itself, applications are unaware of the extended nature of geographically dispersed clusters. There is no topology or configuration information provided to applications to make them aware of the different sites.
Typically, no changes are required to ensure that an application runs, as expected, on a geographically dispersed cluster. However, you should check with the application vendors. In some cases different failure timeout periods may be required since disk accesses and failover times may be longer due to the extended distance between clusters and the need to provide mirroring or replication of data between sites.
Quorum
Q. What is the quorum used for?
A. Server clusters require a quorum resource to function. The quorum resource, like any other resource, is a resource which can only be owned by one server at a time, and for which servers can negotiate for ownership. Negotiating for the quorum resource allows Server clusters to avoid "split-brain" situations where the servers are active and think the other servers are down. This can happen when, for example, the cluster interconnect is lost and network response time is problematic. The quorum resource is used to store the definitive copy of the cluster configuration so that regardless of any sequence of failures, the cluster configuration will always remain consistent.
Q. What options do I have for my quorum resource?
A. In Windows 2000, there are two quorum-capable resources:
· Physical Disk Resource
This allows a disk on the shared cluster storage bus to be used as the quorum resource. The cluster code uses SCSI commands to arbitrate for the quorum disk ensuring only one node owns the quorum disk at any point in time. This is the standard quorum resource that Microsoft recommends for all production Windows 2000 clusters.
Note
Some storage vendors provide their own quorum capable resources for specific hardware solutions (e.g., IBM Shark Storage) or software solutions (such as Veritas Volume Manager). You should use these if they are required for your environment.
· Local Quorum
This quorum-capable resource allows a single node cluster to be setup without having a second storage bus. This type of cluster is useful for developing cluster-aware software without having to have a multi-node cluster.Local Quorum can be used in a production environment if you want to take advantage of the resource health monitoring and local restart facilities provided by Server cluster on a single node.
In Windows Server 2003 we have introduced another quorum capable resource type
Majority Node Set
A majority node set is a single quorum resource from a Server cluster perspective; however, all of the quorum data is actually stored on multiple disks across the cluster. The majority node set resource takes care to ensure that the cluster configuration data stored on the majority node set is kept consistent across the different disks.
See What is Majority Node Set? for more details.
Q. Can other applications share the quorum disk?
A. Microsoft recommends that you do NOT use the quorum disk for other applications in the cluster and that the quorum disk is restricted to use by the cluster service itself. If you use the quorum disk for other applications you should be aware of the following:
· The quorum disk health determines the health of the entire cluster. If the quorum disk fails, the cluster service will become unavailable on all cluster nodes. The cluster service checks the health of the quorum disk and arbitrates for exclusive access to the physical drive using standard I/O operations. These operations are queued to the device along with any other I/Os to that device. If the cluster service I/O operations are delayed by extremely heavy traffic, the cluster service will declare the quorum disk as failed and force a regroup to bring the quorum back online somewhere else in the cluster. To protect against malicious applications flooding the quorum disk with I/Os, the quorum disk should be protected. Access to the quorum disk should be restricted to the local administrator group and the cluster service account.
· If the quorum disk fills up, the cluster service may be unable to log required data. In this case, the cluster service will fail, potentially on all cluster nodes. To protect against malicious applications filling up the quorum disk, access should be restricted to the local administrator group and the cluster service account.
· The cluster service itself will always try to bring the quorum disk back online. In doing so, it may violate the failover and failback policies assigned to applications in the same group.
Q. Can a NAS device be used as the shared quorum disk?
A. No, out-of-the-box, the cluster service supports physical disks on the shared cluster bus or in Windows Server 2003 Majority Node Set quorum resources.
Q. What is Majority Node Set?
A. A majority node set is a single quorum resource from a Server cluster perspective; however, all of the quorum data is actually stored on multiple disks across the cluster. The majority node set resource takes care to ensure that the cluster configuration data stored on the majority node set is kept consistent across the different disks. This allows cluster topologies as follows:
The disks that make up the majority node set could, in principle be local disks physically attached to the nodes themselves or disks on a shared storage fabric. In the majority node set implementation that is provided as part of Server clusters in Windows Server 2003, every node in the cluster uses a directory on its own local system disk to store the quorum data. If the configuration of the cluster changes, that change is reflected across the different disks. The change is only considered to have been committed (i.e. made persistent), if that change is made to:
(/2) + 1
This ensures that a majority of the nodes have an up-to-date copy of the data. The cluster service itself will only start up, and therefore bring resources on line, if a majority of the nodes configured as part of the cluster are up and running the cluster service. If there are fewer nodes, the cluster is said not to have quorum and therefore the cluster service waits (trying to restart) until more nodes try to join. Only when a majority or quorum of nodes, are available, will the cluster service start up the resources be brought online. This way, since the up-to-date configuration is written to a majority of the nodes, regardless of node failures, the cluster will always guarantee that it starts up with the latest and most up-to-date configuration.
Cluster-Aware Applications
Q. What is a Cluster Aware Application?
A. A cluster-aware application is an application that calls the cluster APIs to determine the context under which it is running (such as the virtual server name etc.) and can failover between nodes for high availability.
Can applications that were not written for a cluster be made highly available?
Yes, Server clusters provide a plug-in environment that allows resource dlls to provide the necessary control and health monitoring functions to make existing applications highly available.
Server clusters provide a set of generic resource types that can be used to make existing applications failover in a cluster. In Windows 2000 there are two generic resource types:
Generic application
allows any application to be started, stopped and monitored by the cluster service
Generic Service
allows an existing Windows Service to be started, stopped and monitored.
These generic services provide very rudimentary health monitoring (for example, is the process that was started still a valid process on the system). It does not check that the application is servicing requests since this requires specific knowledge of the application. The generic resources can be used to make applications failover relatively quickly; however, to provide a more appropriate health check, Microsoft recommends that you build an application-specific resource dll.
In Windows Server 2003, we have provided an additional resource type (Generic Script) that allows the start/stop and health monitoring functions to be implemented as scripts rather than using C or C++. This makes the job of building application-specific resource plug-ins much more manageable and easier.
Q. How do I build a cluster-aware application?
A. Server clusters provides a rich API set that allows applications to recognize and utilize the cluster environment. These APIs are fully documented in the Platform SDK.
Q. Should I use the Generic Service or Generic Application resource to make my application highly available?
A. The generic services provide very rudimentary health monitoring (for example, is the process that was started still a valid process on the system). It does not check that the application is servicing requests since this requires specific knowledge of the application. The generic resources can be used to make applications failover relatively quickly; however, to provide a more appropriate health check, Microsoft recommends that you build an application-specific resource dll.
Q. Does Microsoft validate or logo software products that work with Server clusters?
A. Yes, Server clustering is an optional component of the Windows Advanced Server logo program. Applications can be logoed as working on a Server cluster.
Q. What Microsoft Applications are cluster-aware?
A. The following services shipped as part of the Windows operating system are cluster-aware:
· DHCP Highly available DHCP server
· MSDTC Highly available distributed transaction coordinator
· IIS Highly available web server and FTP server*
· File Share Highly available file share service and DFS
· Message Queue Highly available MSMQ service
· MSMQ triggers Highly available MSMQ trigger service (new for Windows Server 2003)
· Print Spooler Highly available printer service
· WINS Highly available WINS service
The following additional Microsoft Products are cluster-aware:
· SQL Server 6.5, 7.0, 2000 and upwards
· Exchange Server 5.5 and upwards
· Services for Unix 3.0 and upwards
Q. Does Exchange 2000 support active/active clusters?
A. Yes, there are some caveats to supporting Exchange 2000 in an active/active configuration.
Q. Does SQL Server support active/active clusters?
A. Yes, SQL Server allows the following:
· Multiple nodes in a cluster hosting different databases. Each database can be failed over independently.
· Multiple nodes in a cluster hosting partitions of a single database using database views to tie the different instances into a single logical database from a client perspective.
Q. Where do I find information about writing cluster-aware applications?
A. The cluster concepts and APIs are fully documented in the Platform SDK. In addition, there are several examples in the Platform SDK that can be used to demonstrate Server cluster integration.
Q. Is Services for Macintosh (SFM) supported in a Server cluster?
A. No, Services for Macintosh is not supported in a Server cluster.
Q. Is Services for Unix (SFU) supported in a Server cluster?
A. Yes, Services for Unix supports highly available NFS shares in SFU 3.0.
Miscellaneous Topics
Q. Can Server clusters and Network Load Balancing be used on the same set of servers?
A. No, Microsoft Server clusters (MSCS) and Network Load Balancing (NLB) are not supported on the same set of nodes. Both Server clusters and Network Load Balancing clusters control and configure network adapters. Since they are not aware of each other, configuring one can interfere with the other.
Q. Can I use antivirus software with a Server cluster?
A. Yes, you should make sure that the vendor has tested their solution in a Server cluster environment. Antivirus software typically layers into the storage stack as a disk driver. This can have an impact on the clusters ability to failover a disk if the driver does not support the required features.
Q. Does Microsoft provide a Distributed Lock Manager (DLM)?
A. No, at this time, Microsoft has no plans to release a distributed lock manager, however, that may change if there is sufficient customer demand for a DLM service.
Note
Do not confuse a distributed lock manager with a cluster file system. A cluster file system can be built in a number of ways, using a lock manager is one of them. However, just providing a lock manager does not solve the cluster file system problem.
Q. Will Microsoft provide a shared disk cluster file system?
A. Microsoft is continually looking at ways to improve the services it provides on the Windows operating system. A shared disk cluster file system is a way to provide a number of attributes in a cluster:
· A single file system namespace visible to all applications on all nodes in the cluster
· High speed access to disks from any node in the cluster
Q. How do Server clusters and Fault tolerant servers relate?
A. Server clusters address a number of availability issues:
· Hardware failures
· Operating system failures
· Application failures
· Site failures
· Operating system and application upgrades
Fault tolerant servers provide an extremely reliable server platform that addresses hardware failures by providing redundancy at the hardware level. In some cases, fault tolerant servers can be used to address site failures.
In and of themselves, fault tolerant servers do not address issues relating to operating system and application monitoring and failure recovery, nor do they address upgrading the operating system or applications without taking the service down.
In summary, Server clusters are about providing high availability, fault tolerant servers provide high reliability. By combining highly reliable, fault tolerant servers with Server clusters, you get the best of both worlds; a set of reliable servers that can provide high availability in the face of operating system and application failures and upgrades.
Q. How do I find Server cluster KB Articles?
A. All cluster KB articles can be found at http://go.microsoft.com/fwlink/?LinkID=538
· All Windows NT 4.0 related articles have keyword "MSCS"
· All Windows 2000 articles have keyword "W2000MSCS"
This should allow easy selection of Server cluster related articles
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