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

[Zhi Yong Wu <wuzhy@linux.vnet.ibm.com>]

On Wed, Aug 31, 2011 at 08:18:19AM +0100, Stefan Hajnoczi wrote:
>Subject: Re: The design choice for how to enable block I/O throttling
> function in libvirt
>From: Stefan Hajnoczi <stefanha@gmail.com>
>To: Adam Litke <agl@us.ibm.com>
>Cc: libvir-list@redhat.com, "Daniel P. Berrange" <berrange@redhat.com>, Zhi
> Yong Wu <wuzhy@linux.vnet.ibm.com>, Zhi Yong Wu <zwu.kernel@gmail.com>
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>On Tue, Aug 30, 2011 at 2:46 PM, Adam Litke <agl@us.ibm.com> wrote:
>> On Tue, Aug 30, 2011 at 09:53:33AM +0100, Stefan Hajnoczi wrote:
>>> On Tue, Aug 30, 2011 at 3:55 AM, Zhi Yong Wu <zwu.kernel@gmail.com> wrote:
>>> > I am trying to enable block I/O throttling function in libvirt. But
>>> > currently i met some design questions, and don't make sure if we
>>> > should extend blkiotune to support block I/O throttling or introduce
>>> > one new libvirt command "blkiothrottle" to cover it or not. If you
>>> > have some better idea, pls don't hesitate to drop your comments.
>>>
>>> A little bit of context: this discussion is about adding libvirt
>>> support for QEMU disk I/O throttling.
>>
>> Thanks for the additional context Stefan.
>>
>>> Today libvirt supports the cgroups blkio-controller, which handles
>>> proportional shares and throughput/iops limits on host block devices.
>>> blkio-controller does not support network file systems (NFS) or other
>>> QEMU remote block drivers (curl, Ceph/rbd, sheepdog) since they are
>>> not host block devices.  QEMU I/O throttling works with all types of
>>> -drive and therefore complements blkio-controller.
>>
>> The first question that pops into my mind is: Should a user need to understand
>> when to use the cgroups blkio-controller vs. the QEMU I/O throttling method?  In
>> my opinion, it would be nice if libvirt had a single interface for block I/O
>> throttling and libvirt would decide which mechanism to use based on the type of
>> device and the specific limits that need to be set.
>
>Yes, I agree it would be simplest to pick the right mechanism,
>depending on the type of throttling the user wants.  More below.
>
>>> 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.

Danial and other maintainers,

If you are available, can you make some comments for us?:)


Regards,

Zhi Yong Wu
>
>Stefan