Re: [linux-lvm] LVM & high system load

["Heinz J. Mauelshagen" <Mauelshagen@sistina.com>]

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@Sistina.com                           +49 2626 141200
                                                       FAX 924446
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