“OnlineThread: SQL Cluster shared data upgrade failed” error (ID 19019) installing Cumulative Update in SQL Server cluster

Last week I was working with a customer on this obscure SQL Server error right after installing Cumulative Update 9 (CU9) on a SQL Server 2008 two-node cluster. As you will see, the root-cause of the problem was easy to understand, but getting to the point of finding this root-cause was not. In this case…

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