What is Log Send Queue, Redo Queue and Redo Rate in Availability Groups?

What is Log Send Queue, Redo Queue and Redo Rate in Availability Groups?

A quick description of 3 metrics that SQL tracks in Availability Groups. These metrics are important when evaluating the health of your Availability Group, and knowing what sort of data loss you might face in a failover. Remember, just because you told SQL to make your AAG Synchronous, doesn’t mean you won’t have data loss.

Log Send Queue

This is how much data is sitting on the primary server that has not yet been replicated to the secondary. In the event of your primary server going offline, this is the amount of data you stand to lose (RPO) and/or can be used to calculate how much time it will take to failover to another server (might affect RTO). You’ll need to manually calculate how long it’ll take to get the data from the primary to the secondary.

Redo Queue

This is how much data is sitting in the log file on the secondary, but is not yet usable by the secondary. If the primary fails, this data is recoverable. Think of it like crash recovery when SQL Server is restarted. This metric will help calculate RTO when combined with the next metric

Redo Rate

This is how quickly the Redo Queue is being processed. If you have a large redo queue and a small redo rate, then RTO is going to take a while. If the Redo Queue is small and this is large in comparison, then the RTO should be pretty quick.