[linux-lvm] LVM & high system load

[Andreas Dilger]

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.

> 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.

Cheers, Andreas
-- 
Andreas Dilger                               Turbolinux filesystem development
http://sourceforge.net/projects/ext2resize/
http://www-mddsp.enel.ucalgary.ca/People/adilger/