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The Voice of the DBA

Steve Jones is the editor of SQLServerCentral.com and visits a wide variety of data related topics in his daily editorial. Steve has spent years working as a DBA and general purpose Windows administrator, primarily working with SQL Server since it was ported from Sybase in 1990. You can follow Steve on Twitter at twitter.com/way0utwest

Brad McGehee talking about tempdb issues

Brad presented a session that tries to help you identify and fix tempdb problems. Performance monitor is your friend. the local disks, avg read and write counters should help you determine if I/O is an issue. Watch for systems where you have >20ms for the averages, this is a gross number, and not an exact measure, but something to be aware of.

Wait states can help you to identify contention for tempdb on allocation structures. Using sys.dm_OS_waiting_tasks, you can look for pagelatches on the pfs, gam, and sgam pages. If you query and find that you have lots of waits for these allocations, you might want to add more tempdb files to help alleviate contention.

There is no reason a DBA should allow a database to run out of space. A quote from Brad and for the most part i agree. You ought to have alerting setup, monitor space, and grow files as needed. There are cases where a runaway event might cause issues, but for the most part, you should be managing space actively.

Optimizing: a variety of suggestions, with the idea that you ought to assume tempdb will be an issue over time. So pre-plan for performance.
- minimize usage. Dont return more rows than you need. Don’t sort data you don’t need to, don’t use order by or distinct if not needed, keep transactions short, index well, avoid temp tables.
- Avoid cursors, especially static or keyset driven cursors.
- avoid LOB columns if you can, consider vertical parittioning.
- avoid table variables
- avoid triggers where you can
- avoid aggregating large sets of data
- avoid snapshot isolation or read committed isolation levels
- if you use sort in tempdb for index rebuilds, schedule it during off hours

You can use these features if you need them, but understand they impact tempdb. Be smart and minimize the usage where you can.

Isolating tempd on separate physical disks can help speed it up, adding RAM might help if sql server can avoid spilling to tempdb, but that depends if you have memory pressure. If you do not have any memory pressure, then you may not get any benefit from adding ram.

SSDs are used for tempdb, but the problem is that tempdb has lots of read.write and can “wear out” an SSd drive.

“The DBA life is about compromise”. I would agree with that. We can’t always do what we want and dont have the money so we need to make tradeoffs.

Preallocate tempdb space, monitor, and then resize as needed. Use IFI for data growth if needed. Note this does not apply to log growth.

Multiple files can help prevent tempdb contention, recommendation is 1/4 to 1/2 the number of cores, up to 8. Note that you want to make all your files the same size.

If you enable TDE, which is a good feature, be aware that tempdb is encrypted. Also be aware that if you remove TDE from all databases, tempdb remains encrypted. Be careful of “testing” TDE on servers.


Filed under: Blog Tagged: SQL Connections, sql server, syndicated

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