Wednesday, January 13, 2010

Defragement in NetApp

Usually we face this problem with our PC and then we defrag our volumes clear temp files and what not; most of the times that solves the problem, though not fully but yes it gets better.

In NetApp though we don’t have to deal with fragmented registry or temp files but due to nature of WAFL file system it gets fragmented very soon, soon after you start overwriting or start deleting and adding the data to volume. So what do you do then?

Well the answer is very simple use ‘reallocate’ command. Yes, this is the defrag tool of NetApp built right in the Ontap OS.

First you have to turn on the reallocation on system with ‘reallocate on’ command. This command turns on the reallocation on system and same way turns off with off switch.

This can be used not only on volumes, infact you can run this on a file, lun or aggregate itself. However I should warn you that optimization of lun may not give you any performance benefit or may get worse, as Ontap doesn’t have any clue what’s in the lun and it’s file system layout.

If you want to run the reallocation only one time you should use -f or -o switch however if you want Ontap to keep a track of your FS and optimize the data when if feels necessary you should control it with –i switch or schedule it with ‘reallocate schedule’ command.

To check current optimization level of volume, you can use ‘reallocate measure -o ’ or if you want to feel adventurous use ‘wafl scan measure_layout ’ through advanced mode, though I don’t suggest using wafl set of commands in general use but yes sometime you want to do something different.

This command is pretty straightforward and no harm (except extra load on CPU and disk) so you can play with this but you should always consider using -p switch for volumes having snapshot and/or snapmirror on to keep the snapshot size small.
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