Re: Very low disk performance on 5.x
From: Eric Anderson (anderson_at_centtech.com)
Date: 05/02/05
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Date: Mon, 02 May 2005 07:40:07 -0500 To: QArne_=5C=22W=F6rner=5C=22=22?= <arne_woerner@yahoo.com>
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. ------------------------------------------------------------------------ _______________________________________________ freebsd-performance@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-performance To unsubscribe, send any mail to "freebsd-performance-unsubscribe@freebsd.org"
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