linux-kernel - Re: Abysmal disk performance, how to debug?

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Date:	Sat, 20 Jan 2007 19:52:53 +0200
From:	Ismail Dönmez <ismail@...dus.org.tr>
To:	Willy Tarreau <w@....eu>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Abysmal disk performance, how to debug?

20 Oca 2007 Cts 19:45 tarihinde şunları yazmıştınız:
[...]
> > vaio cartman # hdparm -tT /dev/hda
> >
> > /dev/hda:
> >  Timing cached reads:   1576 MB in  2.00 seconds = 788.18 MB/sec
> >  Timing buffered disk reads:   74 MB in  3.01 seconds =  24.55 MB/sec
> >
> >
> > [~]> time dd if=/dev/zero of=/tmp/1GB bs=1M count=1024
> > 1024+0 records in
> > 1024+0 records out
> > 1073741824 bytes (1,1 GB) copied, 77,2809 s, 13,9 MB/s
> >
> > real    1m17.482s
> > user    0m0.003s
> > sys     0m2.350s
>
> That's not bad at all ! I suspect that if your system becomes unresponsive,
> it's because real writes start when the cache is full. And if you fill
> 512 MB of RAM with data that you then need to flush on disk at 14 MB/s, it
> can take about 40 seconds during which it might be difficult to do
> anything.
>
> Try lowering the cache flush starting point to about 10 MB if you want
> (2% of 512 MB) :
>
> # echo 2 >/proc/sys/vm/dirty_ratio
> # echo 2 >/proc/sys/vm/dirty_background_ratio

After that I get,

[~]>  time dd if=/dev/zero of=/tmp/1GB bs=1M count=1024
1024+0 records in
1024+0 records out
1073741824 bytes (1,1 GB) copied, 41,7005 s, 25,7 MB/s

real    0m41.926s
user    0m0.007s
sys     0m2.500s


not bad! thanks :)

Regards,
ismail
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