Date: Thu, 22 Oct 2015 13:47:53 +0200
From: Kevin Wolf <kwolf@redhat.com>
Message-ID: <20151022114753.GH3941@noname.redhat.com>
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Subject: Re: [Qemu-devel] [PATCH 0/3] Add 'blockdev-del' command
To: Alberto Garcia <berto@igalia.com>
Cc: qemu-block@nongnu.org, Markus Armbruster <armbru@redhat.com>, qemu-devel@nongnu.org, Max Reitz <mreitz@redhat.com>, Stefan Hajnoczi <stefanha@redhat.com>

Am 22.10.2015 um 13:31 hat Alberto Garcia geschrieben:
> On Thu 22 Oct 2015 01:25:05 PM CEST, Kevin Wolf wrote:
> >> But there's this case that is not so obvious. It involves the new
> >> 'blockdev-snapshot' command I'm working on:
> >> 
> >>   - blockdev-add id=drive0 node-name=node0 file=hd0.qcow2
> >>   - qemu-img create -f qcow2 -b hd0.qcow2 overlay0.qcow2
> >>   - blockdev-add node-name=overlay0 file=overlay0.qcow2
> >>   - blockdev-snapshot node=hd0 overlay=overlay0
> >> 
> >> At this point you have drive0 with overlay0 inserted, and hd0 as its
> >> backing image. All these operation will fail:
> >> 
> >>   - blockdev-del id=drive0         because overlay0 has two references
> >>                                    (monitor and block backend)
> >>   - blockdev-del node=overlay0     for the same reason
> >>   - blockdev-del node=hd0          because it's a backing image
> >> 
> >> In order to delete all this you need to:
> >> 
> >>   - eject device=drive0            overlay0 has one reference left
> >>   - blockdev-del id=drive0
> >>   - blockdev-del node=overlay0     this deletes hd0 as well
> >> 
> >> Does this make sense, or do we need to rethink the semantics a bit more?
> >
> > Well, it's consistent with what you described above.
> >
> > The confusing part might be that you could blockdev-del id=drive0
> > originally, but after taking the snapshot it doesn't work any
> > more. The only way I can see to remove this effect is that you always
> > need to eject the BDS first, even if its only reference is from the BB
> > that is going to be deleted.
> >
> > I guess that would be even clearer rules, but of course it also means
> > that it's a bit more cumbersome to use. If it helps avoiding bugs in
> > management tools, it might be worth it.
> 
> That would be a good reason to force the user to eject the media
> first. Note however that in this case you would still need to delete
> overlay0 manually, as it would still have the monitor reference.

Yes, but I think that's expected because you had a separate blockdev-add.

> However if the snapshot is created using blockdev-snapshot-sync that
> problem does not exist because that extra reference is not there.

Hm... Actually I see a good question here. It's not clear to me that a
BDS created with blockdev-snapshot-sync shouldn't be considered
explicit, especially if a node-name was passed. I guess you can bring up
good arguments for either behaviour.

Kevin