So this week I was patching our SQL Servers that host our availability groups. Never had an issue with them in the past, fairly standard process. I have two servers (call them SQL1 and SQL2) in the cluster and one Availability Group (AG1) uses SQL1 as the primary, and three Availability Groups (AG2-4) uses SQL2 as its primary. They were running v15.0.4013.40, so Server 2019 and this week I had to install CU3 and CU4.
My process was:
- I moved AG1 to use SQL2 as the primary
- Proceed to patch SQL1
- Switch all availability groups to async commit
- Reboot SQL1
- Return all availability groups to sync commit
- Return AG1 to use SQL1 as the primary
In step 2 I patched both CU3 and then CU4 successively. Then, when trying to switch all AGs to sync commit, the databases threw errors in the Availability Group Dashboard and they wouldn't allows access to their databases. I couldn't see any logged reason to why it refused to do this. It was only resolved when I then patched SQL2 with both CUs, and it again allowed to sync commit.
My question is did anyone else have this issue? Was this likely due to me trying to be efficient and patch two CUs in the same interval without doing it one by one? Just trying to work out how I don't do this again as it caused me quite the headache