Re: [Qemu-devel] [PATCH V2 09/12] qmp: add interface blockdev-snapshot-delete-internal-sync

[Stefan Hajnoczi <stefanha@redhat.com>]

On Wed, Jun 19, 2013 at 04:53:38PM +0800, Wenchao Xia wrote:
> 于 2013-6-19 15:46, Stefan Hajnoczi 写道:
> >On Wed, Jun 19, 2013 at 02:18:48PM +0800, Wenchao Xia wrote:
> >>于 2013-6-18 22:20, Stefan Hajnoczi 写道:
> >>>On Mon, Jun 17, 2013 at 11:25:26AM +0800, Wenchao Xia wrote:
> >>>>于 2013-6-15 17:55, Eric Blake 写道:
> >>>>>Should this command be made available via 'transaction'?  That is, if I
> >>>>>have a two-disk VM, and use 'transaction' to take a snapshot of both
> >>>>>disks at once, shouldn't I also have a way to delete the snapshots of
> >>>>>both at once, or gracefully fail without data loss if the second one has
> >>>>>problems?
> >>>>
> >>>>   I think adding it in transaction is not very useful but brings more
> >>>>complexity. Transcation is used to guareentee all operations are taken
> >>>>in one time point, for example, snapshot creation use it to make sure
> >>>>all are consistent to VM. But for deletion, this requirement do not
> >>>>exist.
> >>>
> >>>I guess the problem is: can we make internal snapshot deletion
> >>>transactional?  It's hard to do rollback for snapshot deletion.
> >>>
> >>   Several deletion in transaction equals to several calls of
> >>'blockdev-snapshot-delete-internal-sync', unlike creation, so I hope
> >>not add it which have rollback issue.
> >>
> >>
> >>>But batching is definitely useful for doing 'delvm' in QMP.  I just
> >>>don't think transactions help.  We just need a 'delvm' equivalent in
> >>>QMP.
> >>>
> >>   Maybe the caller can encapsulate a batch interface at its level.
> >
> >'delvm' is a batch interface - it deletes internal snapshots that have
> >the same name across multiple devices.
> >
> >It's not as flexible as:
> >blkdev-internal-snapshot-delete drive0 drive2 drive4
> >
> >Because that would allow you to specify specific drives.
> >
>   Do you mean this interface should be changed as
> blkdev-internal-snapshot-delete devices_array name *id?
> 
>   It seems not much difference with following methods:
> 
> for device in device_list:
>   blkdev-internal-snapshot-delete device name

The ability to batch in a single QMP command feels a little nicer.
There are less opportunities for the operation to stop half-way through.

It would be usable as a QMP 'delvm' and in a more flexible way to delete
multiple internal snapshots.

Would be interesting to see what Eric Blake thinks from a libvirt
perspective.

Stefan