Sql-server – SQL Server full safety mirror commit overhead less then Time to send

mirroringperformancesql serversql-server-2008

How is it possible that the time to send value is greater then the mirror commit overhead? The mirror operates under full safety so this means that the commit is only done @ principal when the commit is successful @ mirror. When the time to send is 2 seconds, this should mean that that overhead is at least 2 seconds. Or am I missing something?

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Best Answer

Database Mirroring Monitor (Status Page):

Time to send log (estimated)
Approximate amount of time the principal server instance requires to send the log that is currently in the send queue to the mirror server instance (the send rate). Because the rate of incoming transactions can vary significantly, the time to send log is an estimate. However, the send rate can be useful for roughly estimating the time required for a manual failover.

The time to send measures the time to send all the pending log. It only applies if there is a pending log (eg. the principal has run exposed and mirror is in 'synchronizing' state). Notice that in this state the principal does not wait for commits to be acknowledged by mirror, hence the commit overhead is significantly lower (1-5 ms) than when running in 'synchronized' state (20-40-80ms).

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