Hi Friends,

SQL Server clustering is an adventure and the devil lies in the details. I am designing a SQL Server High Availability solution and SQL Server Failover clusters are on top of my agenda right now.

Many DBAs (especially newbies) assume clustering to be a one-stop HA solution protecting the server, the instance, the database and the data !!!

Well, that’s not true. There is a lot of misunderstanding relating to what SQL Server Failover clusters can do and what it cannot. So here is a sneak peak on SQL Server clustering limitations:

What can SQL Server Failover Clusters do?

  1. Increases the availability of physical server hardware – In case the power goes down on the box on which the SQL instance is running, a failover happens to another node in the cluster.
  2. Increases the availability of Operating System – In case the OS gets unresponsive; for example after you apply a hotfix or a patch, the OS gets unstable, failover is initiated
  3. Increases the availability of SQL Server Service itself

What can SQL Server Failover Clusters cannot do?

  1. Bring the downtime to zero. DBAs need to be aware of the fact that failover could take a few seconds to a few minutes depending on the amount of work that needs to be re-done and undone on the passive node.
  2. Protect the Data. DBAs should not be surprised here. SQL Server Failover clusters do not protect data as the disks are shared; so the nodes are basically sharing the disk array. In case a disk gets faulty, it’s for all the nodes. DBAs need to implement proper RAID configuration to protect the data.
  3. Load Balancing. SQL Server Failover Clusters is not a load balancing solution.
  4. Also note that just because you have implemented SQL Server Failover Clusters, the front-end applications do not automatically become cluster aware. In many cases the logic to re-try connecting to the new node needs to be implemented.

Amit Bansal
SQL MCM, SQL MVP
Director – eDominer Systems, Peopleware India, SQLMaestros

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