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Always-on Parallel Redo Thread

sakuraime 2,321 Reputation points
2021-01-27T03:00:42.347+00:00

https://learn.microsoft.com/en-us/sql/database-engine/availability-groups/windows/prereqs-restrictions-recommendations-always-on-availability?view=sql-server-ver15

"A SQL Server instance uses up to 100 threads for parallel redo for secondary replicas. Each database uses up to one-half of the total number of CPU cores, but not more than 16 threads per database. If the total number of required threads for a single instance exceeds 100, SQL Server uses a single redo thread for every remaining database. Serial Redo threads are released after ~15 seconds of inactivity."

So if there are 2 and more AGs in a secondary replica , so it still using 100 thread maximum per that SQL instance ?

are there parallel log send thread on primary replica ?

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  1. CathyJi-MSFT 21,131 Reputation points Microsoft Vendor
    2021-01-27T10:06:05.01+00:00

    Hi @sakuraime ,

    >So if there are 2 and more AGs in a secondary replica , so it still using 100 thread maximum per that SQL instance ?

    Yes. A SQL Server instance uses up to 100 threads for parallel redo.

    > are there parallel log send thread on primary replica ?

    No, log send thread is single thread.

    Bes regards,
    Cathy

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