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

From: Osier Yang
Subject: Re: [Qemu-devel] [RFC] block I/O throttling: how to enable in libvirt
Date: Thu, 01 Sep 2011 12:21:20 +0800
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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:
 From the presentation, it seems that both the cgroups method the the qemu 
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
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.

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

And if you meant to post to libvir-list, then s/Danial/Daniel/  :-)


Zhi Yong Wu

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