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

Zhi Yong Wu wuzhy at linux.vnet.ibm.com
Fri Sep 2 02:09:16 UTC 2011

On Thu, Sep 01, 2011 at 09:11:49AM +0100, Stefan Hajnoczi wrote:
>Date: Thu, 1 Sep 2011 09:11:49 +0100
>From: Stefan Hajnoczi <stefanha at gmail.com>
>To: Zhi Yong Wu <wuzhy at linux.vnet.ibm.com>
>Message-ID: <20110901081149.GB14245 at stefanha-thinkpad.localdomain>
>References: <20110901050531.GB17963 at f15.cn.ibm.com>
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>Cc: libvir-list at redhat.com, guijianfeng at cn.fujitsu.com, agl at us.ibm.com,
> qemu-devel at nongnu.org, zwu.kernel at gmail.com, hutao at cn.fujitsu.com
>Subject: Re: [Qemu-devel] [RFC] block I/O throttling: how to enable in
>	libvirt
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>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.
QEMU I/O throttling currently don't support those options of blkiotune, such as --live, --config and --current.If those bps/iops settings are modified, it will immediately take effect.


Zhi Yong Wu

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