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

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Message-Id: <200701202006.45080.ismail@pardus.org.tr>
Date:	Sat, 20 Jan 2007 20:06:44 +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 20:03 tarihinde şunları yazmıştınız:
> On Sat, Jan 20, 2007 at 07:52:53PM +0200, Ismail Dönmez wrote:
> > 20 Oca 2007 Cts 19:45 tarihinde ??unlar?? yazm????t??n??z:
> > [...]
> >
> > > > 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 :)
>
> It is not expected to increase write performance, but it should help
> you do something else during that time, or also give more responsiveness
> to Ctrl-C. It is possible that you have fast and slow RAM, or that your
> video card uses shared memory which slows down some parts of memory
> which are not used anymore with those parameters.

Thanks I will try to upgrade RAM but for now at least responsiveness seems to 
be better.

Regards,
ismail
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