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:

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On Tue, Aug 30, 2011 at 3:55 AM, Zhi Yong Wu <zwu.kernel@gmail.com> wrote:

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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.

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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.

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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:

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http://www.linux-kvm.org/wiki/images/7/72/2011-forum-keep-a-limit-on-it-io-t...

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