Inconsistent IO performance

[Roland Smith]

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

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

Th maximum variation in write speed is (87878728-86397125)/86397125*100% =
1.7%, which doesn't seem that much to me.

This is in multi-user, with X11 running but on an otherwise idling machine,
and with filesystem overhead to boot. Still the numbers are a lot higher than
yours, which puzzles me. 

Trying only reading does yield very inconsistent results because of caching, I
think;

    dd if=/tmp/foo bs=1m count=100 of=/dev/null

1) 1454216957 bytes/sec
2) 1003691691 bytes/sec
3) 1429956761 bytes/sec
4) 2324794646 bytes/sec
5) 1804563681 bytes/sec

This is a (2324794646-1003691691)/1003691691*100% = 132% difference. OTOH,
your data set should be big enough to negate caching effects, I guess. :-)

What this does show is that writing seems to be the bottleneck.

If I both read from and write to a file (on the same disk & partition);

    dd if=/tmp/foo bs=1m count=100 of=/tmp/bar

gives

1) 85161534 bytes/sec
2) 84978770 bytes/sec
3) 87966613 bytes/sec
4) 83036312 bytes/sec
5) 86536879 bytes/sec

This is a (87966613-83036312)/83036312*100% = 5.9% difference between largest
and smallest. The speed seems to be dictated by the writing.

> Can anyone explain what might be causing such a dramatic difference?

Maybe there is a hardware component here? Are both disks on the same
controller? Or if not are both controllers using the same interrupt line?

You should have a look at 'systat -vmstat' with dd running in the
background. That might give a clue as to where the bottleneck is.

Roland
-- 
R.F.Smith                                   http://www.xs4all.nl/~rsmith/
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