[CentOS-virt] Bad Disk Performance of domU

Mathew S. McCarrell mccarrms at gmail.com
Wed May 20 18:24:36 UTC 2009

In general, disk performance on a domU will always be lower than on a dom0.
That being said, I agree with Christopher that is sounds like you are using
a sparse disk file.

Did you create this VM with virt-install?  If so, you can pass the
--nonsparse option to virt-install when creating the VM to allocate all the
disk space when creating the VM.


Mathew S. McCarrell
Clarkson University '10

mccarrms at gmail.com
mccarrms at clarkson.edu

On Wed, May 20, 2009 at 12:35 PM, Francisco Pérez <fperez.x at gmail.com>wrote:

> LVM on dom0 or domu?
> 2009/5/20 Christopher G. Stach II <cgs at ldsys.net>
> ----- "Francisco Pérez" <fperez.x at gmail.com> wrote:
>> > I guess the bad performance it goes with the writing. For example on
>> > decompression of a big tar.gz, installing rpm packages. Also on
>> > restoring a MySQL Database...
>> >
>> > The first clue when i knew that something were wrong was on the FS
>> > formating stage. It took really long time to format 40 GB to ext3. I
>> > did the same on other machine with a sata disk... surprise! same
>> > software and hardware config except for the inferior disk type and did
>> > it in no time.
>> It sounds like your the virtual disk file is sparse.  If that's the case,
>> you traded performance for space.  It not only has to deal with the
>> underlying filesystem, but it allocates the space on demand and hits that
>> filesystem's journal.  Try an LVM logical disk and see how that performs.
>> --
>> Christopher G. Stach II
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