[linux-lvm] LVM & high system load
Heinz J. Mauelshagen
Mauelshagen at sistina.com
Wed May 2 11:08:56 UTC 2001
On Tue, May 01, 2001 at 05:25:36PM -0600, Andreas Dilger wrote:
> Dominique Larchey writes:
> > 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
>
> LVM shows 4.7% overhead. This is not so great.
>
> > 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.
> >
> > 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
>
> Good news - not an LVM problem. Bad news - LVM shows 6.8% overhead.
Did nothing else produce io load while the test on the LV was running?
Did you repeat tests in order to get statistic certainty?
>
> > Perhaps poor perf. for the other person is also DMA related ?-)
>
> No, the other person has drastically different performance with LVM for
> 512-byte I/O compared to raw disk. For 64k-byte I/O performance on LVM
> and raw disk are the same. I _suppose_ that if I/O is done in small
> chunks that PIO mode will have more overhead. Maybe worth asking if
> CPU is high when he is doing 512-byte I/Os.
Yep.
>
> Cheers, Andreas
> --
> Andreas Dilger Turbolinux filesystem development
> http://sourceforge.net/projects/ext2resize/
> http://www-mddsp.enel.ucalgary.ca/People/adilger/
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--
Regards,
Heinz -- The LVM Guy --
*** Software bugs are stupid.
Nevertheless it needs not so stupid people to solve them ***
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Heinz Mauelshagen Sistina Software Inc.
Senior Consultant/Developer Am Sonnenhang 11
56242 Marienrachdorf
Germany
Mauelshagen at Sistina.com +49 2626 141200
FAX 924446
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