Does compression on zfs pool affect performance? .......WAS, Re: Silicon Image SiI 3124 and 3132 RAID controllers

Richard Mahlerwein mahlerrd at
Thu Mar 11 19:08:46 UTC 2010

>----- Original Message ----

>From: Dan Nelson <dnelson at>
>In the last episode (Mar 11), Leslie Jensen said:
>> The solution for me was to create a raidz which gave me the same amount of
>> space.  Now I wonder, should I enable compression?  Will it affect
>> performance?
>The default lzjb compression is very fast and won't consume much CPU.  If
>you have lots of easily-compressabe data it should improve performance.

In informal testing I have found it depends on a lot of factors but is often a net gain.

For instance, writes during testing 
   in a VM running FBSD8 w/ ZFS 
   writing to an attached single test "drive" 
   on my not-terribly-fast XP laptop 
showed a significant improvement when I enabled compression on an FS, jumping from about 5 MB/s to easily 8+ MB/s.

In testing at work on a FBSD8 guest on an ESX3.5 host running on a blade backed by a lightly loaded set of 24 15krpm drives, I found enabling compression didn't change much unless I tossed more than a single processor at it, at which point the gains were more tangible though not spectacular.

I would suspect that it depends on the ratio of the speed of your CPU to the speed of your hard disk subsystem.  Faster CPUs with slower disks will benefit more, slow CPUs with fast disks may even slow down.  Obviously, what constitutes "Fast" and "Slow" is the big sticking point.  :)  


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