Date: Fri, 13 Aug 2010 13:36:12 -1000 From: Clifton Royston <cliftonr@lava.net> To: Roland Smith <rsmith@xs4all.nl> Cc: stable@freebsd.org Subject: Re: Inconsistent IO performance Message-ID: <20100813233612.GB4280@lava.net> In-Reply-To: <20100813213205.GB29150@slackbox.erewhon.net> References: <20100813160109.8BDDA1CC3A@ptavv.es.net> <20100813213205.GB29150@slackbox.erewhon.net>
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On Fri, Aug 13, 2010 at 11:32:05PM +0200, Roland Smith wrote: > On Fri, Aug 13, 2010 at 09:01:09AM -0700, Kevin Oberman wrote: > > For some time I have seen very odd issues with IO performance on > > 8-Stable. Going back to November of last year when 8.0 was released, I > > see variations of up to 22% in identical operations. This is not a > > degradation as the performance moves up and down. > > > > This is a very simplistic case. I have two identical disks (Fujitsu 80G) > > on a ThinkPad T43 with a 2 GHz CPU and 2G RAM. I run the command: > > dd bs=516096 if=/dev/ad0 of=/dev/ad2 > > Why are you using this peculiar block size? > > > Note the dramatic differences even on the same kernel. For the December > > 6 kernel, for example, I see a maximum of 23,676,086 and a minimum of > > just 18,304,565. ???? > > Both figures seem quite low to me? I cannot exactly reproduce your test, > because I don't have an empty second disk handy, but doing > > dd if=/dev/zero bs=1m count=100 of=/tmp/foo With a total write size of 100MB, aren't you just testing speed of writing into cache RAM this way? I think you need to write amounts dramatically greater than the total size of the RAM to get values which appropriately measure disk speed. > yields the following writing speed on 8.1-RELEASE amd64, > WDC WD5001ABYS SATA harddisk @ 7200 rpm.: > > 1) 87263174 bytes/sec > 2) 87878728 bytes/sec > 3) 86397125 bytes/sec > 4) 86550094 bytes/sec > 5) 86524741 bytes/sec This also supports that theory - off the top of my head, maximum theoretical possible write throughput to a similarly sized 7200rpm drive should be 70MB/s (buffer to disk data transfer rate according to WDC's specs.) <http://wdc.com/en/library/sata/2879-701277.pdf>; -- Clifton -- Clifton Royston -- cliftonr@iandicomputing.com / cliftonr@lava.net President - I and I Computing * http://www.iandicomputing.com/ Custom programming, network design, systems and network consulting services
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