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I am upgrading a mirroring pair of SQL2005 Enterprise servers to SQL2008R2 Standard which has 1 thread for mirroring limitation.

As far as I understand this limitation applies to redo queue: http://social.msdn.microsoft.com/Forums/en-US/sqldatabasemirroring/thread/a5b58253-cf4a-4dd6-9472-c484347d39ab/ but I don't fully understand what this means.

My question is: how do I measure the current database system to make sure I won't run into problems with this upgrade (with this 1 thread limitation)?

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Under normal operations, the redo queue is close to zero. What you can do is: track your current redo counters in perfmon (SQLServer:Database Mirroring> Redo Bytes/sec,Redo Queue). With this value you will know how much of your redo log performance will be affected when switching between the SQL editions.

Note that is SQL2008 R2 EE, if you have fewer that five CPUS, SQL will still use only one thread for the redo process. So if you have four or lesser CPUs, SQL will still use a single threaded redo operation.

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Thank you! It is difficult to translate bytes/sec into 1 thread. I have 4 logical CPU with SQL2005 EE - perhaps there is actually no change. – Dina Mar 8 '12 at 21:14

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