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

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Message-ID: <20070120201923.GC25307@1wt.eu>
Date:	Sat, 20 Jan 2007 21:19:23 +0100
From:	Willy Tarreau <w@....eu>
To:	Ismail Dönmez <ismail@...dus.org.tr>
Cc:	Tim Schmielau <tim@...sik3.uni-rostock.de>,
	linux-kernel@...r.kernel.org
Subject: Re: Abysmal disk performance, how to debug?

On Sat, Jan 20, 2007 at 10:16:15PM +0200, Ismail Dönmez wrote:
> 20 Oca 2007 Cts 22:10 tarihinde, Tim Schmielau ??unlar?? yazm????t??: 
> [...]
> >
> > Note that these dd "benchmarks" are completely bogus, because the data=20
> > doesn't actually get written to disk in that time. For some enlightening=20
> > data, try
> >
> >   time dd if=3D/dev/zero of=3D/tmp/1GB bs=3D1M count=3D1024; time sync
> >
> > The dd returns as soon as all data could be buffered in RAM. Only sync=20
> > will show how long it takes to actually write out the data to disk.
> > also explains why you see better results is writeout starts earlier.
> 
> Still not that bad:
> 
> [~]> time dd if=/dev/zero of=/tmp/1GB bs=1M count=1024;sync
> 1024+0 records in
> 1024+0 records out
> 1073741824 bytes (1,1 GB) copied, 53,3194 s, 20,1 MB/s
> 
> real    0m53.517s
> user    0m0.003s
> sys     0m3.193s

No, your measure is wrong because time measures "dd" and sync is done
after. Either use Tim's method (time sync) or the one I proposed in
previous mail (time dd | sync). Anyway, in your situation with a very
small buffer, this should not change by more than half a second or so.

Willy

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