On Wed, Mar 7, 2018 at 10:55 AM, Stefan Hajnoczi <
address@hidden> wrote:
>
> On Tue, Mar 6, 2018 at 11:25 PM, Stefano Panella <
address@hidden> wrote:
> > I have applied this patch and when I run the following qmp commands I I do
> > not see the crash anymore but there is still something wrong because only
> > /root/a is opened from qemu. It looks like nbd-server-stop is also getting
> > rid of the nodes added with blockdev-snapshot-sync, therfore is than not
> > possible to do blockdev-del on /root/d because node-name is not found
>
> Nodes are reference counted. If nothing holds a refcount then the
> node is freed.
Thanks, that explains the behaviour
>
> The blockdev-add command holds a reference to the node. The node will
> stay alive until blockdev-del, which releases that reference.
>
> blockdev-snapshot-sync does not hold a reference. Therefore snapshot
> nodes are freed once nothing is using them anymore. When the snapshot
> node is created, the users of the parent node are updated to point to
> the snapshot node instead. This is why the NBD server switches to the
> snapshot mode after blockdev-snapshot-sync.
>
> This is why the snapshot nodes disappear after the NBD server is
> stopped while /root/a stays alive.
>
> I'm not sure if the current blockdev-snapshot-sync behavior is useful.
> Perhaps the presence of the "snapshot-node-name" argument should cause
> the snapshot node to be treated as monitor-owned, just like
> blockdev-add. This would introduce leaks for existing QMP clients
> though, so it may be necessary to add yet another argument for this
> behavior.
that would be nice, I mean to add an extra parameter so it is added to the monitor
>
> Anyway, I hope this explains the current behavior. I don't see a
> problem with it, but it's something the API users need to be aware of.
>
Yes, I was not aware of that behaviour, the problem is that many examples refer
to having a device associated with the blockdev-add'd node therefore we do not
see this problem.
> If it is a problem for your use case, please explain what you are trying to do.
>
It is not strictly a problem for my usecase but it would be nice to have the extra param to
blockdev-snapshot-sync. That would also fix the problem of running multiple snap-sync
after blockdev-add but before there is any user.
> Stefan