Difference between Data file shrinking and Log file shrinking

Posted: October 29, 2012 by Virendra Yaduvanshi in Database Administrator
Tags: , ,

You may be confused to find this question’s answer, after so lots of google, you may read that shrinking data files is good, and in other places you may read that it’s bad.

A shrink operation on a data file tries to move the database page nearest the end of the file down toward the start of the file. This creates “empty” space at the end of the data file that can be returned to the OS. In other words, the data file is made physically smaller.

A shrink operation on a transaction log file, on the other hand, doesn’t move anything around—it simply removes the empty portion of the transaction log at the end of the file, as long as the transaction log records are not being retained for any reason. If the operation is successful, the log file is made physically smaller.

The confusion comes from the side effects of the two operations, and when they should be performed.

People are advised (or decide) to shrink data files to reclaim space. It may be that their index maintenance job causes the data files to grow, or their drive is getting full, and the natural reaction is to reclaim some of this “wasted” space. However, it is likely this space will be needed again and often it’s better to leave the remaining free space available to the data file rather than repeatedly shrink and auto-grow a file.

Shrinking a data file should be a very rare operation because it has a nasty side effect. Shrinking a data file causes massive index fragmentation, which may affect query performance. For same you may read Paul S. Randal blog post “Why You Should Not Shrink Your Data Files” includes a simple script that shows this.

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