Re: [Qemu-devel] Live migration using qcow2

[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 invalidating that cache, you might (partially) read outdated data.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.

Paolo