For Availability Groups we have a few extra error numbers we care about. Error number 1480 tells when a server changes roles, so we can know when a server flips from a secondary to a primary, or from a primary to a secondary. Error number 35264 tells when data movement has suspended on any database. This can occur for many reasons. One I have seen is when you have expanded your mount point on your primary and the data or log file runs out of space on the secondary the data or log file can not expand on the secondary because you forgot to expand the secondary. Error number 35265 tells you when the data movement has resumed on any database. Error number 41404 let’s you know if your AG is offline which can be bad if you expected an automatic failover. Error number 41405 let’s you know if an Availability Group can’t automatically failover for any reason. In the later to cases you will want to look at your SQL Error logs and AlwaysOn Extended Events Health session.
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