On Tue, Nov 24, 2009 at 11:03 AM, Richard Elling
<richard.ell...@gmail.com> wrote:
> Try disabling prefetch.
Just tried it... no change in random read (still 17-18 MB/sec for a
single thread), but sequential read performance dropped from about 200
MB/sec. to 100 MB/sec. (as expected). Test case is a 3 GB file
accessed in 256 KB records. ARC is set to a max of 1 GB for testing.
arcstat.pl shows that the vast majority (>95%) of reads are missing
the cache.
The reason I don't think that this ishitting our end users is the
cache hit ratio (reported by arc_summary.pl) is 95% on the production
system (I am working on our test system and am the only one using it
right now, so all the I/O load is iozone).
I think my next step (beyond more poking with DTrace) is to try a
backup and see what I get for ARC hit ratio ... I expect it to be low,
but I may be surprised (then I have to figure out why backups are as
slow as they are). We are using NetBackup and it takes about 3 days to
do a FULL on a 3.3 TB zfs with about 30 million files. Differential
Incrementals take 16-22 hours (and almost no data changes). The
production server is an M4000, 4 dual core CPUs, 16 GB memory, and
about 25 TB of data overall. A big SAMBA file server.
--
{--------1---------2---------3---------4---------5---------6---------7---------}
Paul Kraus
-> Senior Systems Architect, Garnet River ( http://www.garnetriver.com/ )
-> Sound Coordinator, Schenectady Light Opera Company (
http://www.sloctheater.org/ )
-> Technical Advisor, Lunacon 2010 (http://www.lunacon.org/)
-> Technical Advisor, RPI Players
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