[dm-devel] Re: I/O bandwidth control on KVM

[Anthony Liguori]

Hi Ryo,

Ryo Tsuruta wrote:
> Hello all,
> 
> I've implemented a block device which throttles block I/O bandwidth, 
> which I called dm-ioband, and been trying to throttle I/O bandwidth on
> KVM environment. But unfortunately it doesn't work well, the number of
> issued I/Os is not according to the bandwidth setting.
> On the other hand, I got the good result when accessing directly to
> the local disk on the local machine.
> 
> I'm not so familiar with KVM. Could anyone give me any advice?

If you are using virtio drivers in the guest (which I presume you are 
given the reference to /dev/vda), try using the following -drive syntax:

-drive file=/dev/mapper/ioband1,if=virtio,boot=on,cache=off

This will force the use of O_DIRECT.  By default, QEMU does not open 
with O_DIRECT so you'll see page cache effects.

Regards,

Anthony Liguori