Re: [Qemu-block] [PATCH v9 2/3] quorum: implement bdrv_add_child() and bdrv_del_child()

[Alberto Garcia]

On Fri 22 Jan 2016 09:02:10 PM CET, "Dr. David Alan Gilbert" <address@hidden> 
wrote:

>> >>>> In general, what do you do to make sure that the data in a new
>> >>>> Quorum child is consistent with that of the rest of the array?
>> >>>
>> >>> Quorum can have more than one child when it starts. But we don't
>> >>> do the similar check. So I don't think we should do such check
>> >>> here.
>> >> 
>> >> Yes, but when you start a VM you can verify in advance that all
>> >> members of the Quorum have the same data. If you do that on a
>> >> running VM how can you know if the new disk is consistent with the
>> >> others?
>> >
>> > User error if it is not.  Just the same as it is user error if you
>> > request a shallow drive-mirror but the destination is not the same
>> > contents as the backing file.  I don't think qemu has to protect us
>> > from user error in this case.
>> 
>> But the backing file is read-only so the user can guarantee that the
>> destination has the same data before the shallow mirror. How do you
>> do that in this case?
>
> I think in the colo case they're relying on doing a block migrate to
> synchronise the remote disk prior to switching into colo mode.

Yes but this is a general API that can be used independently from
COLO. I'd say if we want to allow that we should at least place a big
warning in the documentation.

Berto