[fedora-virt] tuning fdatasync for kvm?

[Adam Huffman]

On Wed, Apr 14, 2010 at 7:04 AM, Dor Laor <dlaor at redhat.com> wrote:
> On 04/13/2010 05:33 PM, Bill McGonigle wrote:
>> Hi, all,
>>
>> The subject line is a bit of a guess.  I'm running the preview release
>> on an updated F12, with a Vista guest (no virtio drivers yet) and after
>> an initial OS install the software updates have been running for a bit
>> more than 12 hours, and the system's hard drives have been thrashing
>> heartily during it.
>>
>> I've got a data-journaled ext4 on luks on raid-1.  That's clearly not a
>> write-optimized stack, but performance has been pretty good in the past
>> with KVM&  Vista and fine for normal system operations.  If I check out
>> iotop while it's thrashing it's all in [jbd2/dm-2-8] and [kdmflush].
>>
>> Looking for the blocked tasks (below), I think I see kvm is
>> fdatasync'ing, on what I'd presume is a very frequent basis.  I noticed
>> that fsync was replaced with fdatasync not too long ago, but that should
>> have helped performance, not clobbered it (I think...).  Ideally I could
>
> Adding Christoph who has done the change.
> Do you use cache=off or writethrough?
>
>> tell kvm to fdatasync every 5 seconds or something like that and get
>> batched writes.  I've tried switching schedules to cfq, deadline, and
>> noop, with no big difference.
>>
>> Does this seem sensible or am I totally off-base here?  I'm rebuilding
>> this virtual machine after a virtio driver install went south, so I'd
>> like to at least have a usable solution (but not libeatmydata) without
>> them.  I also run OS's with no virtio driver support at times.
>>

Not sure if it's related but I've been seeing very poor IO performance
for guests on a fairly new F12 system.  It doesn't seem to make a
difference whether the guests are Linux or Windows.  For instance, an
XP installation is running right now and qemu-kvm is struggling along
only occasionally writing around 35KB/s, according to iotop.  A Centos
5.4 installation I did last week was also somewhat hamstrung by poor
virtual disk performance.  In both cases this is with image files
rather than logical volumes.  I know the latter if preferable but I
haven't seen this kind of slowdown with image files before.

Adam