Re: [Qemu-devel] Live migration using qcow2

[Tiziano Müller]

Am Mittwoch, den 20.02.2013, 15:16 +0100 schrieb Paolo Bonzini:
> Il 20/02/2013 14:46, Kevin Wolf ha scritto:
> >> > 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 invalidating that cache, you might (partially) read outdated data.
> 
> Also, note that some backends (iSCSI and NBD for example) will never use
> a cache on the host, so they can be used with the default
> cache=writeback mode.
> 
> However, this is simply because writeback is mapped to none, and
> writethrough to directsync.  If you used iSCSI or NBD through the
> kernel, you would need cache=none.

We are using GlusterFS. Currently with a FUSE-based mount with qcow2s on
it but switching to qemu-GlusterFS-devices later. But thanks for the
clarifications.

Tiziano

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