From: Manfred Hollstein (manfred_at_die-hollsteins.de)
Date: Mon Dec 10 2007 - 12:01:21 CET
Date: Mon, 10 Dec 2007 12:01:21 +0100 From: Manfred Hollstein <manfred@die-hollsteins.de> Message-ID: <20071210110121.GA6017@saturn.hollstein.homelinux.org> Subject: Re: [suse-sles-e] Very high MD latencies with SLES10 SP1.
Hi there,
I forwarded your message to Neil Brown, who's the MD maintainer at SUSE
amongst other things. Here's his reply:
(I'm not on this list, but this message was forwarded to me...)
On Wednesday December 5, noe@physik.unizh.ch wrote:
> Hi,
>
> we're having a bit of a performance problem with our MD RAID1 setup after
> having switched to SLES10 SP1 (from SLES9 SP3). For random-I/O in particular,
> we're seeing maximum read and write latencies up to _several seconds_ (with averages
> around 30ms) where SLES9 showed maxima around 500ms with avg around 20ms.
>
> Typical outputs (randomio-1.3, using O_DIRECT):
>
> sles9sp3:
> total | read: latency (ms) | write: latency (ms)
> iops | iops min avg max sdev | iops min avg max sdev
> --------+-----------------------------------+----------------------------------
> 636.8 | 476.7 0.2 20.6 501.6 39.1 | 160.1 0.4 1.0 25.0 1.4
>
> sles10sp1:
> total | read: latency (ms) | write: latency (ms)
> iops | iops min avg max sdev | iops min avg max sdev
> --------+-----------------------------------+----------------------------------
> 350.1 | 262.5 0.2 29.9 7114.5 284.6 | 87.6 0.5 22.0 6511.6 253.6
This looks bad. I've tried on may machine and I cannot reproduce it,
so presumably drive subsystem as a significant effect (I've got some
fairly boring SATA drives on a cheap SATA card).
I presume you are running 'randomio' on a file that you created in the
XFS partition - is that correct?
Could you try failing one half of the mirror so that all the requests go
just to the one drive?
Also, would it be possible to make a single drive into a RAID0. That
might help show if the problem is specific to raid1, or if it affect
md more generally.
NeilBrown
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