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How People Abuse Their SQL Server Transaction Log – Things NOT to Do

Care and Feeding of the Transaction Log

Want to learn more about how the transaction log works? Kalen Delaney, celebrated author of SQL Server 2008 Internals and the Inside SQL Server series as well as a world class trainer and performance tuner, 聽is giving a pre-conference talk at SQL PASS next week. The pre and post conference sessions are full day sessions you can purchase as an add-on to the regular conference (which runs Tuesday-Thursday). Kalen’s pre-conference session “Care and Feeding of the Transaction Log” is scheduled for Monday, November 2, 2009. She will be raffling off copies of her book and will have copies of her DVD to give away. If you are guilty of any of the transaction log practices below, or have colleagues who聽are and you need ammunition in your fight for change, Kalen’s session will be a great help to you! Note that she will not necessarily be covering all the items below, they are my own list and not hers. 馃檪

How People Abuse Their SQL Server Transaction Log – Things NOT to Do

聽聽聽聽聽聽聽聽 Rely on autogrow – this causes fragmentation, poorly allocated VLFs, a performance hit for duration of grow, etc.

聽聽聽聽聽聽聽聽 Don’t take backups at all or do them on a schedule that doesn’t meet SLAs or doesn’t keep the log “small enough”.

聽聽聽聽聽聽聽聽 Use simple recovery mode in the belief it will improve performance when there are no bulk inserts or something else that actually benefits from minimal logging.

聽聽聽聽聽聽聽聽 Want to “turn off logging”.

聽聽聽聽聽聽聽聽 “Shrink” the log by deleting the LDF file – can result in corruption and an unusable database.

聽聽聽聽聽聽聽聽 Shrink the log file when it will just grow again.

聽聽聽聽聽聽聽聽 Add multiple LDF files due to the mistaken impression that this will improve performance (multiple threads myth or thinking it will split IO over multiple files when logs are mostly sequential). This is particularly a problem with TempDB where we recommend 1/4 to 1 file per core and don’t always make it clear we mean only the data files and not the log file.

聽聽聽聽聽聽聽聽 Only backing up the log files when they reach a certain percentage full to reduce the number of backups occurring at any one time – this can result in not being able to meet your recovery SLAs.

聽聽聽聽聽聽聽聽 Enable instant file initialization and expect it to help with log growth.

聽聽聽聽聽聽聽聽 Create one extremely large log file without regard to # of VLFs created.

聽聽聽聽聽聽聽聽 Sharepoint documents have recommended simple recovery mode to “keep the log from filling” but don’t explain the tradeoffs for recoverability such as losing Point in Time recovery options.

聽聽聽聽聽聽聽聽 Don’t understand the difference between truncating and shrinking the log.

聽聽聽聽聽聽聽聽 Don’t realize that in simple mode you still have to do a full backup first.

聽聽聽聽聽聽聽聽 Don’t realize they need to do a log backup after switching from simple or bulk logged to full.

聽聽聽聽聽聽聽聽 Think DBCC CHECKDB checks for inconsistencies in the log

聽聽聽聽聽聽聽聽 Put data and log on same drive (mix random and mostly sequential), put lots of logs on one drive (defeats purpose of having a log/sequential writes on own IO path), put output of profiler or other activity on same drive as a log

聽聽聽聽聽聽聽聽 Don’t put log on fast enough/properly configured drive (RAID 10, disk partition alignment, separate from file server/other SQL/db data, disk allocation unit = 64k, HBA queue depth 64+, anything that gets writes to 3ms or less, etc.)

聽聽聽聽聽聽聽聽 Ignore corruption messages/event log messages about bad IO

聽聽聽聽聽聽聽聽 Don’t secure the directories where the MDF/LDF and backup files reside

聽聽聽聽聽聽聽聽 Ignore the fact that Tran replication affects log size (can’t truncate until log reader has read data from log)

聽聽聽聽聽聽聽聽 Do many transaction log backups between full backups (with few or no differentials) and don’t consider what happens if one of the files is corrupted/missing or how long it will take to do the restores

聽聽聽聽聽聽聽聽 Don’t test the DR strategy, or at least have it documented and planned/thought out

聽聽聽聽聽聽聽聽 Don’t exclude the MDF/LDF files from virus scanning software

Instead of abusing your transaction log, pay attention to Pond’s Twelfth Law: Don鈥檛 practice in front of the CIO.聽 A professional prepares ahead of time. To help you prepare, attend Kalen’s pre-conference session!