Re: FW: cgroup blkio.weight working, but not for KVM guests

[Stefan Hajnoczi <stefanha@gmail.com>]

On Mon, Oct 22, 2012 at 07:36:34AM -0600, Ben Clay wrote:
> Forwarding this to the KVM general list.  I doubt you folks can help me with
> libvirt, but I was wondering if there’s some way to verify if the cache=none
> parameter is being respected for my KVM guest’s disk image, or if there are
> any other configuration/debug steps appropriate for KVM + virtio + cgroup.

Here's how you can double-check the O_DIRECT flag:

Find the QEMU process PID on the host:

ps aux | grep qemu

Then find the file descriptor of the image file which the QEMU process
has open:

ls -l /proc/$PID/fd

Finally look at the file descriptor flags to confirm it is O_DIRECT:

grep ^flags: /proc/$PID/fdinfo/$FD

Note the flags field is in octal and you're looking for:

#define O_DIRECT        00040000        /* direct disk access hint */

Stefan