On Wed, 8 Jul 2009, Yaroslav Tykhiy wrote:
My conclusion is that although ZFS prefetch is supposed to be adaptive and
handle random access more or less OK, in reality there is plenty of room for
improvement, so to speak, and for now Postgresql performance can benefit from
its staying just disabled. The same may apply to other database systems as
well.
Yup; this is even spelled out at
http://www.cuddletech.com/blog/pivot/entry.php?id=1040
"...the most common complain tends to be by databases which strictly work
in fixed 8K blocks and manage their own caches very effectively. If you
think you have such a case, file-level prefetch can be tuned on the fly
using mdb, I encourage you to play with it and see what is best for your
workload..."
Anecdotal reports (which never seem to have repeatable test cases sadly)
abound about prefetch issues:
http://southbrain.com/south/2008/04/the-nightmare-comes-slowly-zfs.html
http://unix.derkeiler.com/Mailing-Lists/FreeBSD/current/2007-06/msg00671.html
Also, there was a pretty serious ZFS problem in this area that got fixed
in the middle of last year on Solaris. Your FreeBSD install might be
based on a build that is using the older, known bad logic here. See
http://www.solarisinternals.com/wiki/index.php/ZFS_Evil_Tuning_Guide#Device-Level_Prefetching
and http://bugs.opensolaris.org/bugdatabase/view_bug.do?bug_id=6437054 for
details.
--
* Greg Smith gsmith@xxxxxxxxxxxxx http://www.gregsmith.com Baltimore, MD
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