The IO scheduler improvements I saw

August 31, 2009

In the spirit of sharing actual numbers and details for things that I left a bit unclear in an earlier entry, here is more:

First, we switched to the deadline IO scheduler (from the default cfq). I did brief tests with the noop scheduler and found it basically no different from deadline for my test setup, and deadline may have some advantages for us with more realistic IO loads.

My IO tests were sequential read and write IO, performed directly on a test fileserver, which uses a single iSCSI backend. On a ZFS pool that effectively is a stripe of two mirror pairs, switching the backend to deadline increased a single sequential read from about 175 MBytes/sec to about 200 Mbytes/sec. Two sequential reads of separate files were more dramatic; aggregate performance jumped by somewhere around 50 Mbytes/sec. In both cases, this was close to saturating both gigabit connections between the iSCSI backend and the fileserver.

(Since all of these data rates are well over the 115 Mbytes/sec or so that NFS clients can get out of our Solaris fileservers, this may not make a significant difference in client performance.)

I measured no speed increase for a single sequential writer, but it was already more or less going at what I believe is the raw disk write speed. (According to the IET mailing list, other people have seen much more dramatic increases in write speeds.)

I didn't try to do systematic tests; for our purposes, it was enough that deadline IO scheduling had a visible performance effect and didn't seem to have any downsides. I didn't need to know the specific contours of all of the improvements we might possibly get before I could recommend deployment on the production machines.

Written on 31 August 2009.
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Last modified: Mon Aug 31 01:27:53 2009
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