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.