Re: [Qemu-devel] Can qemu reopen image files?

[Eric Blake <eblake@redhat.com>]

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On 12/28/2016 12:51 PM, Christopher Pereira wrote:
> Hi Eric,
> 
> There is something I don't understand.
> 
> We are doing: "virsh save", "qemu-img convert", "qemu-img rebase" and
> "virsh restore".
> We only touch the backing chain by doing the rebase while the VM is down.
> Is there any chance this procedure can destroy data?

Since there are never two active writers to the file (you have made sure
that qemu-img and qemu are not both holding the same file open at once),
there is no chance for corruption on that front.  There's still a
possibility of corruption if you botch the instructions, and don't
rebuild the chain in such a way that the guest would see the same data,
but that's harder to say without seeing an actual transcript.

> If so, is there any difference between shutting down and just
> saving/restoring the VM?

Shutting down vs. saving/restoring shouldn't matter other than the
length of time that the guest is offline (including rebooting time with
a shut down, which is not present for an incoming migration restoration).

> Maybe save/restore keeps a cache?

No. Both shutting down and saving end the qemu process, and therefore
flush everything to the disk.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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