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Rolling Windows Upgrades with AGs + WSFC

Allan Hirt shows how you can combine Availability Groups with Windows Server Failover Clusters and upgrade the operating system version while keeping your SQL Servers running:

The configuration for a cluster rolling upgrade allows for mixed Windows Server versions to coexist in the same WSFC. This is NOT a deployment method. It is an upgrade method. DO NOT use this for normal use. Unfortunately, Microsoft did not put a time limit on how long you can run in this condition, so you could be stupid and do something like have a mixed Windows Server 2012 R2/2016 WSFC. Fire, ready, aim. The WSFC knows about this and you’ll see a warning with an Event ID of 1548.

Read on for a summary of what Allan has learned in doing this.