SQL Server 2008 : Failover clustering - Clustering overview

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As a cluster-aware application, a SQL Server instance can be installed into an existing Windows cluster, creating what's referred to as a failover clustering instance. Once installed, the instance is accessed using a network name (aka virtual server name) without needing to know which of the underlying physical cluster servers the instance is currently running on.

The abstraction between a physical and a virtual server is a key component in providing continued database availability after a failure event. In the event of a server failure, the SQL Server instance automatically moves, or fails over, to another cluster server, while continuing to be accessed using the same virtual server name.

In this section, we'll take a high-level look at clustering architecture, including its benefits, limitations, and common usage scenarios.

1. Clustering architecture

Built on top of a Windows Server failover cluster, SQL Server clustering uses the shared nothing architecture, a term used to distinguish its clustering implementation from those of other database vendors, such as Oracle's Real Application Cluster (RAC). Under the shared nothing architecture, a SQL Server database instance is active on only one physical clustered server at any given time.

Unlike a network load-balanced solution, SQL Server failover clustering isn't a high-performance solution; in other words, the performance won't be any better, or worse, than a nonclustered implementation. SQL Server clustering is purely a high-availability solution and you should not choose it for any other reason.

Figure 1 illustrates a simple example of a two-node cluster with a single SQL Server failover clustering instance. Ordinarily, the instance resides on the Sales1 server. In the event of a failure of this server, the SQL Server instance automatically fails over to the Sales2 server. In either case, the SQL instance will continue to be accessed using the same virtual server name.

Figure 1. A simple failover clustering example in which a SQL Server instance can move from one cluster server to another in the event of failure

Key components of a Windows Server failover cluster solution are shared storage and resource arbitration. In figure 5.1, we can see both the Sales1 and Sales2 servers have access to shared storage. The databases contained in the SQL instance reside on the shared storage to enable either server to access them when required. But in order to prevent both servers from accessing them at the same time, and therefore causing data corruption, the Windows clustering service arbitrates ownership of the disk volumes based on the server's current role in the cluster.

Clusters consist of one or more resource groups, which are collections of resources required by a clustered application. In the case of a clustered SQL Server instance, its resource group would include the disks containing the database instance's data and transaction logs, an IP address and network name, and the three application services: SQL Server, SQL Agent, and full-text service. Failover is a term used to describe the transfer of ownership of resources from one server to another; it occurs at the resource group level, ensuring that all resources required for the application are moved and available on the failover server.

Given that a database instance can be owned by various servers in the cluster, applications are configured to connect to the virtual server name rather than the name of a physical cluster server. To avoid confusion with the Microsoft Virtual Server product, SQL Server 2005 and above refer to failover clustering instances rather than the virtual server name known in SQL Server 2000 and earlier.

The terms and concepts we've covered thus far will become clearer, particularly when we walk through an installation of a clustered instance. For now, let's turn our attention to the major advantages and limitations of SQL Server clustering.

2. SQL Server clustering advantages and limitations

As a high-availability technology, clustering has a number of advantages and limitations when compared to other SQL Server high-availability options such as database mirroring and transaction log shipping . Its primary advantage is that in the event of a server failure, the entire instance and all of its databases are moved to a failover server, a process that usually takes no more than about 90 seconds. This stands in contrast to mirroring or log shipping solutions, which are established on an individual database-by-database basis.

In addition to providing protection from unexpected server failure, the other major benefit of clustering is the ability to reduce downtime during planned outages. For example, consider a requirement to upgrade a server's RAM. Unless we have a server with hot-add RAM capabilities, we'd need to power off the server, upgrade the RAM, and power back on again, and during this time the database instance would be unavailable. In a clustering scenario, we could manually initiate failover, which would move the instance from one server to the other, enabling the upgrade to occur while the instance is available on another cluster server. Thus, the downtime is limited to the failover time, typically about 1 minute.

Unlike database mirroring and transaction log shipping, the major limitation of clustering, particularly in Windows Server 2003 and earlier, is that other than a RAID solution, there's no protection from failure of the disks containing the database files and/or the cluster quorum resource, discussed shortly. Further, typical clustering solutions don't offer protection from geographic disasters—that is, if all clustered servers exist in a single location, the destruction of that location will result in an outage. Although multisite clusters and the new Windows Server 2008 quorum models (discussed shortly) address these limitations, it's a common practice to combine multiple high-availability solutions to deliver the advantages of each while minimizing the individual limitations. For example, important databases within a clustered SQL Server instance can be mirrored to an offsite location for geographic (and disk) protection. 

3. Clustering in Windows Server 2008

A SQL Server failover clustering instance is installed on, and in some ways is constrained by, the underlying Windows cluster. Although Windows Server 2003 and earlier had a solid and full-featured clustering solution, there were a number of limitations that constrained the configuration of the installed SQL Server failover clustering instances.

The SQL Server 2008 release was preceded by the release of Windows Server 2008, bringing with it a substantial improvement in the clustering options. In comparison to Windows Server 2003, the clustering improvements in Windows Server 2008 include the following:

  • An enhanced validation test, as shown in figure 2, which can be used to ensure the validity of the hardware and software components in forming a cluster

  • Support for IPv6 and up to 16 cluster servers (increased from 8)

    Figure 2. Use the Windows Server 2008 Validate a Configuration Wizard to ensure the validity of the cluster components.
  • The ability of cluster servers to obtain IP addresses via DHCP

  • Support for new quorum models, used to determine the number of failures a cluster can sustain while still operating

Apart from the enhanced validation and management tools, the increase in supported server nodes and relaxed networking restrictions enables the creation of much more flexible clustering solutions, making Windows Server 2008 the best server operating system for a clustered SQL Server 2008 deployment. Further enhancing its appeal are the improved quorum models.

Failover Cluster Configuration Program (FCCP)

Cluster hardware should be certified by both the hardware vendor and Microsoft as cluster compatible. With the release of Windows Server 2008, Microsoft announced the Failover Cluster Configuration Program (FCCP). Hardware vendors will certify complete cluster configurations against this program, making the process of choosing a cluster-compatible hardware solution much simpler than in the past.

4. Quorum models

A fundamental aspect of Windows Server clustering is the process whereby each node is assigned unambiguous ownership of a resource. For example, consider a two-node cluster like the one you saw in figure 1. If the network link between the two cluster nodes temporarily drops, what process prevents both servers from assuming that they're now the owner of the SQL instance? This process to resolve such an occurrence (commonly called the split brain problem) is referred to as the cluster quorum.

In earlier versions of Windows clustering (prior to Windows Server 2003), cluster quorum was maintained using a shared disk resource and a quorum database containing the resources and owners. In our previous example, a link failure between the two nodes would be resolved by only one of the nodes having ownership of the quorum disk. When the link is dropped, the node with quorum disk ownership continues its role and takes on the roles (if any) of the other node.

Despite the simplicity and effectiveness of this quorum model, the quorum disk was a single point of failure. Further, given the need for all cluster nodes to have access to the shared disk resource containing the quorum disk, the constraints of typical shared storage hardware prevented the creation of geographically dispersed clusters. Windows Server 2003 addressed this with the introduction of the Majority Node Set (MNS) quorum.

Nodes in an MNS quorum cluster operate with local copies of the quorum database, avoiding the limitations of shared storage, but in order to prevent the split brain problem, a majority of nodes must remain in contact for the cluster to be considered valid. One of the key attributes of an MNS cluster is the requirement for a minimum of three nodes to form a valid cluster, and in the case of a five-node cluster, a majority of the nodes (three) would need to remain in contact for the cluster to continue operating.

Figure 3. Available quorum models in Windows Server 2008

Windows Server 2008 further enhances the quorum model with a voting system. As you can see in figure 3, a number of options are available, with a recommended option based on the number of cluster nodes.

In summary, the following general recommendations apply to the selection of a quorum model in 2008:

  • Node Majority—Used for clusters with an odd number of nodes

  • Node and Disk Majority—Used for clusters with shared storage and an even number of cluster nodes

  • Node and File Share Majority—Designed for multisite and Microsoft Exchange clusters

  • No Majority—Compatible with earlier quorum models; isn't recommended given the single point of failure on the quorum disk

Before we look at some of the more important preinstallation configuration tasks, let's take a moment to dig a little deeper and review common clustering topologies.

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