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

Zhi Yong Wu wuzhy at linux.vnet.ibm.com
Fri Sep 2 02:14:48 UTC 2011

On Thu, Sep 01, 2011 at 01:05:31PM +0800, Zhi Yong Wu wrote:
>----- Forwarded message from Zhi Yong Wu <wuzhy at linux.vnet.ibm.com> -----
>Date: Thu, 1 Sep 2011 11:55:17 +0800
>From: Zhi Yong Wu <wuzhy at linux.vnet.ibm.com>
>To: Stefan Hajnoczi <stefanha at gmail.com>
>Cc: "Daniel P. Berrange" <berrange at redhat.com>, Stefan Hajnoczi
>	<stefanha at gmail.com>, Adam Litke <agl at us.ibm.com>, Zhi Yong Wu
>	<wuzhy at linux.vnet.ibm.com>, QEMU Developers <qemu-devel at nongnu.org>,
>	guijianfeng at cn.fujitsu.com, hutao at cn.fujitsu.com
>Subject: [RFC] block I/O throttling: how to enable in libvirt
>Message-ID: <20110901035517.GD16985 at f15.cn.ibm.com>
>References: <CAEH94Li_C=BOe2gV8NyM48njYWMBAo9MTGc1eUOh-Y=cODs6VA at mail.gmail.com>
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>	<20110830134636.GB29130 at aglitke.rchland.ibm.com>
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>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 at gmail.com>
>>To: Adam Litke <agl at us.ibm.com>
>>Cc: libvir-list at redhat.com, "Daniel P. Berrange" <berrange at redhat.com>, Zhi
>> Yong Wu <wuzhy at linux.vnet.ibm.com>, Zhi Yong Wu <zwu.kernel at gmail.com>
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>>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:
>>>> On Tue, Aug 30, 2011 at 3:55 AM, Zhi Yong Wu <zwu.kernel at 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
>>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.
>Do you agree with introducing one new libvirt command blkiothrottle now?
>If so, i will work on the code draft to make it work.
>Daniel and other maintainers,
>If you are available, can you make some comments for us?:)
HI, Adam,
Now stefan, Daniel, and Gui all suggest extending blkiotune to keep libivrt unified interface. What do you think of it?


Zhi Yong Wu
>Zhi Yong Wu
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