Clustering or Mirroring?

Although SQL Server 2005 SP1 introduces support for mirroring (originally promised for the initial release of SQL Server 2005), the product also continues to support Microsoft Cluster Services (MSCS) as a database failover solution. Although both mirroring and clustering increase database availability via redundancy, the technologies differ in their implementation details, and organizations will need to weigh several factors to determine which method best meets their needs. For some applications, database mirroring may not perform as well as an MSCS-based solution, but it is simpler and less costly and will likely be the preferred solution for most customers and scenarios. However, for databases storing nonrelational data or for high-performance or high-load systems, clustering may be preferable.

Specific areas of difference include the following:

Redundancy and failover speed. Failover strategies based on clustering can achieve higher levels of redundancy than database mirroring strategies. SQL Server 2005 Enterprise Edition supports up to eight redundant nodes in a cluster. Database mirroring, in contrast, is limited to a redundant pair of database servers. However, failover with mirroring is faster. According to Microsoft, automated failover with mirroring should occur in several seconds; failover with clustering will take several times longer.