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

From: Dr. David Alan Gilbert
Subject: Re: [Qemu-devel] [PATCH v9 2/3] quorum: implement bdrv_add_child() and bdrv_del_child()
Date: Fri, 22 Jan 2016 20:02:10 +0000
User-agent: Mutt/1.5.24 (2015-08-30)

* Alberto Garcia (address@hidden) wrote:
> On Thu 21 Jan 2016 05:58:42 PM CET, Eric Blake <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.


> Berto
Dr. David Alan Gilbert / address@hidden / Manchester, UK

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