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

[Zhi Yong Wu]

On Thu, Sep 01, 2011 at 12:21:20PM +0800, Osier Yang wrote:
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>Cc: Stefan Hajnoczi <address@hidden>, Adam Litke <address@hidden>, QEMU
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>Subject: Re: [Qemu-devel] [RFC] block I/O throttling: how to enable in
>       libvirt
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>于 2011年09月01日 11:55, Zhi Yong Wu 写道:
>>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<address@hidden>
>>>To: Adam Litke<address@hidden>
>>>Cc: address@hidden, "Daniel P. Berrange"<address@hidden>, Zhi
>>>Yong Wu<address@hidden>, Zhi Yong Wu<address@hidden>
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>>>
>>>On Tue, Aug 30, 2011 at 2:46 PM, Adam Litke<address@hidden>  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<address@hidden>  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?:)
>
>Seems you posted to wrong list? This should be discussed in libvir-list
>instead.
No, libvirt-list is where i want to post.
>
>And if you meant to post to libvir-list, then s/Danial/Daniel/  :-)
Sorry, thanks for your correcting, Jintao. We have not met for several years.
>
>>
>>Regards,
>>
>>Zhi Yong Wu
>>>Stefan
>
>