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

["Daniel P. Berrange" <berrange@redhat.com>]

On Tue, May 31, 2011 at 10:10:37AM -0400, Vivek Goyal wrote:
> 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).

No, it isn't about putting limits on a disk independant of a VM. It is
about one VM having multiple disks, and wanting to set different policies
for each of its virtual disks. eg

  qemu-kvm -drive file=/dev/sda1 -drive file=/dev/sdb3

and wanting to say that sda1 is limited to 10 MB/s, while sdb3 is
limited to 50 MB/s.  You can't do that kind of thing with cgroups,
because it can only control the entire process, not individual
resources within the process.

Daniel
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