Re: Abysmal disk performance, how to debug?

[Willy Tarreau <w@1wt.eu>]

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.

Regards,
Willy