Always-on : Log Send Queue

Question

THere is a performance counter for Always-on Log Send Queue

The amount of log records in the log files of the primary database, in kilobytes, that are not yet sent to the secondary availability replica.

which means it shows only the data lost for async replica ?

Answer 1

which means it shows only the data lost for async replica ?

You can say that because primary will not wait for the commit on secondary and if primary goes down and these logs are not sent to secondary and you being up secondary this information would not be there and that would be a data loss to you. That is one of the prime disadvantage of Async mode

Answer 2

Hi @sakuraime ,

In case of synchronous replicas, large Send queue implies potential blocking. In the synchronous mode, the commit towards the application is delayed until an acknowledge from the synchronous secondary replicas return and the acknowledge is sent when the content sent to the replica is persisted in the transaction log of secondary replica.

With asynchronous replicas, large Send queue dictates potential data loss – data from the queue has not been sent to secondaries.

https://aboutsqlserver.com/2019/06/09/hadr-sync-commit/

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