Date: Sat, 14 Aug 2010 05:37:10 +0200 From: Erik Trulsson <ertr1013@student.uu.se> To: Clifton Royston <cliftonr@lava.net> Cc: Roland Smith <rsmith@xs4all.nl>, stable@freebsd.org Subject: Re: Inconsistent IO performance Message-ID: <20100814033710.GA96114@owl.midgard.homeip.net> In-Reply-To: <20100813233612.GB4280@lava.net> References: <20100813160109.8BDDA1CC3A@ptavv.es.net> <20100813213205.GB29150@slackbox.erewhon.net> <20100813233612.GB4280@lava.net>
next in thread | previous in thread | raw e-mail | index | archive | help
On Fri, Aug 13, 2010 at 01:36:12PM -1000, Clifton Royston wrote: > 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. One should get much higher speeds if it was just writes into RAM. > > > 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>; That the disks are the same size and RPM is fairly irrelevant for max transfer rate. Bit density on the platters is far more importtant. If you look at the data sheet for that particular model <http://www.wdc.com/wdproducts/library/SpecSheet/ENG/2879-701176.pdf>; the transfer rate is actually specified as 98MB/s. (Presumably along the outer tracks of the disk -- on the inner tracks you probably can't get more than around 50 MB/s) -- <Insert your favourite quote here.> Erik Trulsson ertr1013@student.uu.se
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?20100814033710.GA96114>