[linux-lvm] LVM & high system load
Dominique LARCHEY-WENDLING
Dominique.Larchey-Wendling at loria.fr
Tue May 1 21:50:15 UTC 2001
Andreas Dilger wrote:
> There is one other person that has reported very slow LVM performance.
> However, there is no obvious reason why they have performance issues,
> and others are unable to reproduce this problem.
>
> Can you please try the following and report results:
>
> time dd if=/dev/mmedia/music of=/dev/null bs=512
> time dd if=/dev/mmedia/music of=/dev/null bs=65536
> time dd if=/dev/hda1 of=/dev/null bs=512
> time dd if=/dev/hda1 of=/dev/null bs=65536
Here are the tests I have done ... I use the command line
dd if=/dev/sda of=/dev/null bs=1k count=500k
to fill up the buffer cache before any timing measure.
I point out that /dev/mmedia/music resides on /dev/hda5
time dd if=/dev/mmedia/music of=/dev/null bs=512
count=200000 0m37.113s
time dd if=/dev/hda5 of=/dev/null bs=512
count=200000 0m35.437s
Ok I am ridiculous ... this has nothing to do with LVM.
It has to do with DMA set off by default by linux 2.4.
This is the reason of poor IO perf. and CPU eating.
(as you might have understood, I usually do not use IDE drives)
After DMA is set on,
time dd if=/dev/mmedia/music of=/dev/null bs=512
count=200000 0m12.934s
time dd if=/dev/hda5 of=/dev/null bs=512
count=200000 0m12.110s
Sorry for anoying you with my stupidity ...
Perhaps poor perf. for the other person is also DMA related ?-)
Thanks anyway
DL
--
Dominique Larchey
LORIA, Nancy, France
Tel. (Work) +33 (0) 3 83 59 20 13
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