SQL Server 2008 : Failover clustering - Clustering topologies and failover rules

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There are many aspects to consider when designing and planning high availability with SQL Server clustering. Chief among them is the number of dedicated passive, or standby, servers to include in the cluster for failover purposes.

In simple clusters with a 1:1 working:standby ratio, each working server is matched with a standby server that, during normal operations, isn't used for any purpose other than to simply wait for a failure (or planned failover). Alternatively, both (or all) servers can be active, but this introduces resource complications during failovers; for example, will a server be able to handle its own load plus that of the failed server?

In previous versions of clustering, Active/Passive, or Active/Active terminology was used to define the usage of two-node server clusters. The current terms are single-instance or multi-instance and are more appropriate terms considering the ability of today's clusters to contain up to 16 servers and many more SQL instances, with one or more standby servers available for failover from any number of other active servers.

This section will focus on the various clustering topologies commonly used and the resource considerations for each.

1. Single-instance clusters

Under normal operating conditions, one node is in the standby status, existing purely to take on the load in the event of failure or a planned outage. When such action occurs, the instance moves between one server and the next, as shown in figure 1.

Figure 1. In the event of failure (or planned failover), single instance clusters such as this one have a simple failover process, with an instance moving from the active/failed node to the standby.

This clustering topology, known as a single-instance cluster, is the simplest to understand and administer, and also provides the highest availability while avoiding any performance impacts after failover. As such, it's commonly used to protect mission-critical SQL Server applications.

The major downside of this topology is the cost. During normal operation, one server isn't used in any capacity, and depending on the cost of the server, this can be an expensive approach. In addressing this, the multi-instance topology is frequently used.

2. Multi-instance clusters

As the name suggests, a multi-instance cluster contains multiple SQL Server instances. In a typical two-node multi-instance cluster, each cluster node runs one or more instances, and a failover situation causes an instance to assume another node's workload in addition to its own. In the example in figure 2, the Sales SQL instance resides on the Sales1 server, and the Marketing SQL Instance resides on the Sales2 server. In the event of failure of Sales1, Sales2 will run both the Sales and Marketing instances, potentially reducing the performance of both instances.

Because of the increase in server utilization, multi-instance clusters are typically used in budget-constrained environments, or in those valuing high availability much higher than reduced performance in a failed state.

Figure 2. A two-node multi-instance cluster. Each node needs to be capable of handling the load of both instances during failover scenarios.

The multi-instance example in figure 2 achieves higher resource utilization than the single-instance example presented earlier; both servers are utilized in the normal working status.

Multi-instance clusters require careful consideration in terms of resource configuration. In our example, if the Sales1 and Sales2 servers each have 32GB of RAM, and both the Sales and Marketing SQL instances are using 28GB of RAM, a failover situation would see one (or both) instances with a potentially drastically reduced memory allocation after failover, a scenario I refer to as a resource crunch.

To avoid this issue, configure instances so that the sum total of maximum resource usage across both (or all) instances doesn't exceed the total resources of a single node that the instances could end up running on.

The two topologies we've covered so far represent the opposite ends of the scale; single-instance clusters with a 50 percent node utilization, and multi-instance clusters with 100 percent resource utilization. In between these two lies the N+1/M cluster.

3. N+1/M clusters

To avoid the cost of idle servers and limit the effects of a failover-induced resource crunch, a commonly used cluster configuration is an N+1/M cluster, whereby one or more standby servers exist for more than one working server. For example, in a three-node cluster, two nodes may be active, with a third existing as a failover node for both active nodes. Similarly, a five-node cluster with three active nodes and two failover nodes is a common cluster configuration.

As the number of cluster nodes and SQL Server instances increases, so too does the importance of considering the failover rules governing which node(s) a SQL Server instance can fail over to.

4. Failover rules

By regularly polling cluster nodes using a series of mechanisms called LooksAlive and IsAlive checks, a Windows cluster may conclude that a cluster node has failed. At that point the resources hosted by the node are failed over to another cluster node.

In a simple two-node cluster, the failover process is straightforward, as demonstrated in figure 5.4. But consider a five-node cluster containing two passive/standby nodes. Which (if any) of the standby nodes will be used for failover purposes when one of the active nodes fails? In large clusters containing many SQL Server instances, this is a particularly important consideration in maintaining a well-balanced cluster, ensuring that a single node doesn't carry a disproportionate burden of load.

There are several common strategies and techniques used in controlling failover, and central to them is the use of the Preferred Owners, Possible Owners, and Failback settings.

The properties of the SQL Server resource shows the Possible Owners property. This property lets you specify which nodes the instance is permitted to fail over to. In a similar manner, the Preferred Owner setting (not shown) is used to set the preferred failover node (this may not be chosen in some situations—for example, if the node is unavailable).

Finally, the Failback options (not shown) let you determine whether or not resources fail back to the original node if the failed node comes back online.

With this background in mind, let's continue by walking through the installation of a clustered SQL Server instance into a two-node Windows Server 2008 cluster.

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