openstack team mailing list archive

[Scott Moser <smoser@xxxxxxxxxx>]

On Mon, 26 Mar 2012, Martin van Wilderen - JDN BV wrote:

> Hi Scott,
>
> kvm --version
> QEMU emulator version 0.13.91 (qemu-kvm-0.14.0-rc1), Copyright (c) 2003-2008 Fabrice Bellard
>
> virsh --version
> 0.8.8
>
> uname -r
> 2.6.35-22-server
>
> Need any more info?

Well, unfortunately there is lots more info needed, and I'm not really
able to dedicate resources to debugging this at this point.  I was curious
because I had circumstantial evidence pointing to qemu-kvm 1.0 (in ubuntu
12.04) being slower than 11.10.  However, your data above is similar to
what I've seen in 12.04.

Most definitely, if you want write performance, you will need fully
allocated disks.  The qcow sparse is good for quick deployment, but not
good for write speed.

You may also want to try using qed format disk as that is supposed to be a
bit better.  I had one kvm person suggest looking at barriers of the
host's ext4 filesystem, saying:
  if you have ext4 filesystem underneath with barriers enabled, then
  you'll find that the guest will submit barriers to the host filesystem
  which will end up flushing the device cache

A quick google showed
http://blog.smartlogicsolutions.com/2009/06/04/mount-options-to-improve-ext4-file-system-performance
on generally how to get some additional performance out of ext4.

I'm interested in hearing what you find out, but unfortunately at the
moment, I dont' have a magic bullet for you.


>
> Met vriendelijke groet,
>
> Martin van Wilderen
> Technical Manager
>
> On Mar 26, 2012, at 5:12 PM, Scott Moser wrote:
>
> > Can you provide information about what version of kvm you're using ?
> > I suspect this is really unrelated to openstack specifically.
> >
> > On Mon, 26 Mar 2012, Martin van Wilderen - JDN BV wrote:
> >
> >> Hi List,
> >>
> >> I have a question about KVM disk performance. We are using Openstack Nova on three machines. These machines have a SSD drive with a dd write performance of about 130mb/s
> >>
> >> Within the instance the write performance is down to about 5 mb/s. When using the allocate trick (dd zero to disk before newfs) we get a performance of 20 mb/s.
> >>
> >> Things a have tried but don't give any extra results are:
> >> - Settings the disklayout from qcow2 to raw
> >> - Settings the cache type in libvirt.xml (writeback, writethrough, none)
> >> - Switching KSM on and off.
> >> - Tested with different guests OS, Linux, FreeBSD, Windows.
> >>
> >> Is there someone who had some extra info i can check? Or are there more people with this issue?
> >>
> >> Snippet from libvirt.xml
> >> <driver type='qcow2'/ cache='writeback'/>
> >> <source file='/var/lib/nova/instances/instance-00000113/disk'/>
> >> <target dev='vda' bus='virtio'/>
> >>
> >> Kind regards,
> >>
> >> Martin
> >> _______________________________________________
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> >>
> >>
>
>