Re: [Qemu-devel][RFC]QEMU disk I/O limits

[Vivek Goyal <vgoyal@redhat.com>]

On Tue, May 31, 2011 at 02:56:46PM +0100, Daniel P. Berrange wrote:
> On Tue, May 31, 2011 at 09:45:37AM -0400, Vivek Goyal wrote:
> > On Mon, May 30, 2011 at 01:09:23PM +0800, Zhi Yong Wu wrote:
> > > Hello, all,
> > > 
> > >     I have prepared to work on a feature called "Disk I/O limits" for qemu-kvm projeect.
> > >     This feature will enable the user to cap disk I/O amount performed by a VM.It is important for some storage resources to be shared among multi-VMs. As you've known, if some of VMs are doing excessive disk I/O, they will hurt the performance of other VMs.
> > > 
> > 
> > Hi Zhiyong,
> > 
> > Why not use kernel blkio controller for this and why reinvent the wheel
> > and implement the feature again in qemu?
> 
> The finest level of granularity offered by cgroups apply limits per QEMU
> process. So the blkio controller can't be used to apply controls directly
> to individual disks used by QEMU, only the VM as a whole.

So are multiple VMs using same disk. Then put multiple VMs in same
cgroup and apply the limit on that disk.

Or if you want to put a system wide limit on a disk, then put all
VMs in root cgroup and put limit on root cgroups.

I fail to understand what's the exact requirement here. I thought
the biggest use case was isolation one VM from other which might
be sharing same device. Hence we were interested in putting 
per VM limit on disk and not a system wide limit on disk (independent
of VM).

Thanks
Vivek