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

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Date:	Sat, 20 Jan 2007 21:21:32 +0100 (CET)
From:	Tim Schmielau <tim@...sik3.uni-rostock.de>
To:	Ismail Dönmez <ismail@...dus.org.tr>
cc:	Willy Tarreau <w@....eu>, linux-kernel@...r.kernel.org
Subject: Re: Abysmal disk performance, how to debug?

On Sat, 20 Jan 2007, Ismail Dönmez wrote:

> 20 Oca 2007 Cts 22:10 tarihinde, Tim Schmielau şunları yazmıştı: 
> >
> > 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
> 

That's not the point, you still measured the same as before (but you might 
have noticed that, after printing the results, the shell prompt took some 
time to appear). I appended "time sync" to the command to show that 
(depending on the amount of available memory) actually most of the time 
is spent in the "sync", not the "dd".

Tim