SQL 2005 SP2 maintenance plan bug masking corruption

Not much to do here in Tokyo while waiting for the Bangkok flight except read and blog, and I've already done enough reading for one day on the flight here – so that leaves blogging!

Here's an interesting case that I got involved in on SQLServerCentral (here's the original thread, linked here with permission of the original poster). The problem was that every night the maintenance job would run and end up producing a stack dump, from a 211 error (which always indicates metadata corruption causing the Engine to trip over). I guessed system table corruption but the consistency checking portion of the maintenance job was apparently running fine. So I asked for one of the stack dumps, the output from the maintenance plan, and the latest SQL error log to have a look at (I have a weakness for these things, as you well know).

After having a look, I saw some strange behavior in the error log – every night there 10 or so DBCC CHECKDBs of master within about 2 minutes – corresponding to the consistency checking part of the maintenance job that was clearing running DBCC CHECKDB against all the system and user databases. Then I remembered that vanilla 2005 SP2 has two nasty maintenance plan bugs – one of which caused the consistency checking part of the maintenance plan to malfunction when run, and run DBCC CHECKDB against master every time instead of the database specified. Fixing that would then lead to the being able to properly run consistency checks and find the corrupt database.

So – moral of the story is to make sure you're not running vanilla 2005 SP2 (or one of the builds close to it, 3150 to 3158) otherwise you could get into problems with maintenance plans – worse still, you may think you've been running consistency checks all this time on your user databases, but if no-one's checking the SQL error logs, you could just be repeatedly consistency checking master.

The KB that describes the bug is 934459 (available at http://support.microsoft.com/kb/934459), or you can install SP3 (available at http://support.microsoft.com/?kbid=955706).

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