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Database autogrow and slow database recovery – t-sql Tuesday 21

10 August,2011 by Jack Vamvas

 The theme for T-SQL Tuesday 21 is reveal yoTSQLWednesdayur crap to the world

Like anyone , we all have significant amounts to share,  particularly if you work in a busy production environment.  Deadlines are tight,  business users want everything yesterday which all amounts to decreasing standards of quality-control.  “You live and learn” ……one recent example 

  A nightly ETL process included the  following steps:

 1)       Full Backup of a database

2)       Copy the database to another drive

3)       Restore the database onto another sql server instance and run transitions

 After a large maintenance job on Sundays the transaction  log file physical size of the database in Step 1 would be large.

 Regular  BACKUP LOG commands  during the day managed  the inactive part of  the  Logs.  To decrease the transaction log physical size,   a  DBA applied DBCC SHRINKFILE every night on the log file , prior to step 1.  Auto-grow on the Log file was set 1 MB .

The subtle problem started appearing at step 3. I noticed the Recovery process was taking longer than expected.  

I noticed the DBCC SHRINKFILE  had stopped running a couple of weeks ago.  

Using DBCC LOGINFO(mydb)  - I noticed there were thousands of   Virtual Log Files (VLFs).  A large amount of VLFs can cause the discovery layer on a database recovery process to slow down.

 A transaction log file is composed of VLFs.  When a transaction log file is truncated , the basic unit is a VLF.

The auto-grow mentioned earlier , at 1 MB size was causing an excessive amount of VLFs.  I changed the auto-grow to 100 MB – and  this resolved the situation.  I forced a recovery process with the new autogrow setting , and the recovery process was significantly quicker.

From the account above ,  I‘d highlight two mistakes:

1) Autogrow set at a very low  file size level.  Size the log files accordingly , and try to minimise the amount of times autogrow occurs.   

2)  Shrinking and growing the transaction log files causes file system fragmentation.  This decreases performance. Minimise shrinking of transaction logs

Read More

SQL Server faster restores with instant file initialisation

Storage Checklist FAQ - SQL Server DBA

SQL LOG FILE AUTOGROW performance ... - SQL Server DBA


Author: Jack Vamvas (http://www.sqlserver-dba.com)

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