Re: [Qemu-devel] Live migration using qcow2

[Kevin Wolf]

On Wed, Feb 20, 2013 at 02:14:47PM +0100, Tiziano Müller wrote:
> Am Mittwoch, den 20.02.2013, 12:20 +0100 schrieb Kevin Wolf:
> > On Wed, Feb 20, 2013 at 11:47:56AM +0100, Tiziano Müller wrote:
> > > Hi everyone
> > > 
> > > According to http://wiki.qemu.org/Migration/Storage section "Image
> > > Formats" qemu can't do live migration without data corruption when using
> > > qcow2 or qed due to the metadata caches.
> > > Wasn't that fixed by commit 06d9260 ?
> > 
> > Yes, it is fixed. Depending on your backend, you still need
> > cache=none/directsync, of course.
> 
> Thanks for the fast reply. Can you please elaborate on that (or post a
> link to an explanation): What exactly does the backend have to provide
> that for example cache=writeback can be used? Is there a way to test for
> it?

The problem is about cache coherency. Local files work just fine with
cache=writeback, but if you migrate to a different host, you get the
problem that the destination kernel may cache some parts of the image
while the source is still writing to it. When you do the actual switch
to the destination host and the backend doesn't take care of
invalidating that cache, you might (partially) read outdated data.

Kevin