What is REDO queue in database mirroring?

The REDO queue is the set of log records that have been received on the mirror from the principal, but have not yet been replayed in the mirror database. Once a failure is initiated, the mirror database will not come online as the principal until the REDO queue has been processed. It is very important to monitor the REDO queue on the mirror. The amount of REDO queue correlates to the amount of downtime you’ll experience during a mirroring failover. This is due to the fact that the amount of unrestored log waiting in the redo queue is an indicator of the time required to fail over to the mirror database.

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