[libvirt] [RFC] block I/O throttling: how to enable in libvirt

[Stefan Hajnoczi]

On Thu, Sep 01, 2011 at 01:05:31PM +0800, Zhi Yong Wu wrote:
> On Wed, Aug 31, 2011 at 08:18:19AM +0100, Stefan Hajnoczi wrote:
> >On Tue, Aug 30, 2011 at 2:46 PM, Adam Litke <agl at us.ibm.com> wrote:
> >> On Tue, Aug 30, 2011 at 09:53:33AM +0100, Stefan Hajnoczi wrote:
> >>> I/O throttling can be applied independently to each -drive attached to
> >>> a guest and supports throughput/iops limits.  For more information on
> >>> this QEMU feature and a comparison with blkio-controller, see Ryan
> >>> Harper's KVM Forum 2011 presentation:
> >>
> >>> http://www.linux-kvm.org/wiki/images/7/72/2011-forum-keep-a-limit-on-it-io-throttling-in-qemu.pdf
> >>
> >> From the presentation, it seems that both the cgroups method the the qemu method
> >> offer comparable control (assuming a block device) so it might possible to apply
> >> either method from the same API in a transparent manner.  Am I correct or are we
> >> suggesting that the Qemu throttling approach should always be used for Qemu
> >> domains?
> >
> >QEMU I/O throttling does not provide a proportional share mechanism.
> >So you cannot assign weights to VMs and let them receive a fraction of
> >the available disk time.  That is only supported by cgroups
> >blkio-controller because it requires a global view which QEMU does not
> >have.
> >
> >So I think the two are complementary:
> >
> >If proportional share should be used on a host block device, use
> >cgroups blkio-controller.
> >Otherwise use QEMU I/O throttling.
> Stefan,
> 
> Do you agree with introducing one new libvirt command blkiothrottle now?
> If so, i will work on the code draft to make it work.

No, I think that the blkiotune command should be extended to support
QEMU I/O throttling.  This is not new functionality, we already have
cgroups blkio-controller support today.  Therefore I think it makes
sense to keep a unified interface instead of adding a new command.

Stefan