Date: Mon, 02 May 2005 07:40:07 -0500 From: Eric Anderson <anderson@centtech.com> To: =?ISO-8859-15?Q?=22Arne_=5C=22W=F6rner=5C=22=22?= <arne_woerner@yahoo.com> Cc: Robert Watson <rwatson@freebsd.org> Subject: Re: Very low disk performance on 5.x Message-ID: <42761FA7.1030500@centtech.com> In-Reply-To: <20050501112351.4184.qmail@web41205.mail.yahoo.com> References: <20050501112351.4184.qmail@web41205.mail.yahoo.com>
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Arne Wörner wrote: > --- Robert Watson <rwatson@FreeBSD.org> wrote: > >>On Sat, 30 Apr 2005, Arne WXrner wrote: >> >>>3. The man page geom(4) of R5.3 says "The GEOM framework >>> provides an infrastructure in which "classes" can per- >>> form transformations on disk I/O requests on their path >>> from the upper kernel to the device drivers and back. >>> >>>Could it be, that geom slows something down (in some boxes the >>>reading >>>ops are very slow; in my box the writing ops are very slow)? >> >>[...] >>against a further offset region of ad0. If they're against the >>same bits of disk, the main difference here will be the >>additional processing of the layers in the stack. A little >>bit of math is required to figure out the offset, but dd >>should be usable to figure out the incremental cost. >> > > I used the following commands: > 1. dd if=/dev/ad0s2a bs=16k count=10000 of=/dev/null > 2. dd if=/dev/ad0 iseek=2100357 bs=16k count=10000 of=/dev/null > (I think I did the math correctly; indeed the read speed of my ad0 > varies between 45MB/sec (iseek=0) and 25MB/sec (in the end) with > bs=128k). > The results were nearly the same (both between 26MB/sec and > 28MB/sec). Maybe I should have done it in single user mode. > > My other hard disc ad1 (it is newer and bigger and faster and more > furious) behaves better (but I can just try writing via the file > system (ufs+s) and a final sync): The sync just needs .24sec of > 18.28 seconds; the read and write speed is nearly the same (about > 37MB/sec+/-1MB/sec). > > Is there anything else, that could help to find the reason for the > difference between ad0's speed in R4.11 and R5.3? I'll be honest here, I don't care much if the speed difference between 4.X and 5.X is measureable, or whatever. What I find is a little telling of an issue somewhere, is that READS are slower than WRITES! This is totally bogus to me - dd'ing a file to a filesystem, then umounting should take longer than dd'ing from the disk to /dev/null, it nearly every config I can dream up. Maybe it's the speed at which /dev/null can gobble bits (seems highly unlikely!), or maybe GEOM is busy doing a check or some routine to data being accessed directly from the disk device instead of through a filesystem? I don't know, but it is an issue, and I'm sure we'll get nailed up to a fence in some benchmark somewhere if we don't fix it.. Eric -- ------------------------------------------------------------------------ Eric Anderson Sr. Systems Administrator Centaur Technology A lost ounce of gold may be found, a lost moment of time never. ------------------------------------------------------------------------
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