-> REDO Queue size has reached 12 GB and estimated data loss seems to be close to 22 Minutes.
-> Executed below query to check the REDO thread sessionid on Secondary replica,
SELECT db_name(database_id) as DBName, session_id FROM sys.dm_exec_requests WHERE command = 'DB STARTUP' GO
-> Checked on the secondary if there were any sessions blocking the redo thread,
select * from sysprocesses where blocked>0
-> It seems like SPID 172 is blocking the redo thread. SPID 172 is a select statement. I tried terminating spid 172, but was able to see another select statement taking up its place.
-> There were several Read-Only queries running on secondary replica and these select queries seems to be blocking the redo thread.
-> On the primary replica, Application team is executing a huge write query on a object called Table that is pushing the REDO thread on secondary replica to work hard and harden the data to database.
-> The redo thread on Secondary replica acquires schema modification (Sch-M) locks on the secondary database to make any DDL changes that are performed on primary replica. On the secondary replica, the read-only queries acquire schema stability (Sch-S) locks to process the query. In our case the redo thread is not able to acquire schema modification (Sch-M) locks as the select queries has already acquired schema stability (Sch-S) locks and this is the reason why we were experiencing blocking.
-> We decided to stop the Read-only queries on secondary replica as the operation on primary was more important and also dont want to get into a data loss situation in an event a failover is required.
-> Always on availability group is configured with “Readable Secondary” set to Yes,
-> We decided to change “Readable Secondary” to NO for time being till the REDO queue catches up.
-> Once above change was completed, as expected there were no select queries on secondary replica and the REDO thread started to move forward and there were no blockings.
-> Downside to this change is that all currently executing select queries on the secondary replica will be terminated and also any new read-only workload will have to utilize the primary replica and this could increase the load on Primary replica. All these risks were communicated with application team and a decision was taken to change “Readable Secondary” to NO.
-> Within 15 minutes Redo queue size was under control and we changed “Readable Secondary” back to YES as the write operation on Primary replica also completed fine.
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