Re: [Qemu-devel] [PATCH v2] qapi: converted commit

[Luiz Capitulino <lcapitulino@redhat.com>]

On Thu, 14 Jun 2012 17:21:44 +0200
Pavel Hrdina <phrdina@redhat.com> wrote:

> On 06/14/2012 05:04 PM, Eric Blake wrote:
> > On 06/14/2012 08:56 AM, Pavel Hrdina wrote:
> >> On 06/14/2012 02:18 PM, Eric Blake wrote:
> >>> On 06/14/2012 01:35 AM, Pavel Hrdina wrote:
> >>>> Signed-off-by: Pavel Hrdina<phrdina@redhat.com>
> >>>> ---
> >>>> +++ b/qapi-schema.json
> >>>> @@ -1169,6 +1169,21 @@
> >>>>    { 'command': 'block_resize', 'data': { 'device': 'str', 'size':
> >>>> 'int' }}
> >>>>
> >>>>    ##
> >>>> +# @commit
> >>>> +#
> >>>> +# Commit changes to the disk images (if -snapshot is used) or
> >>>> backing files.
> >>>> +#
> >>>> +# @device: the name of the device or the "all" to commit all devices
> >>>> +#
> >>>> +# Returns: nothing on success
> >>>> +#          If @device is not a valid block device, DeviceNotFound
> >>>> +#          If a long-running operation is using the device, DeviceInUse
> >>>> +#
> >>>> +# Since: 1.2
> >>>> +##
> >>>> +{ 'command': 'commit', 'data': { 'device': 'str' }}
> >>> Should we use this as an opportunity to make the command more powerful?
> >>>    For example, integrating this with the 'transaction' command or a block
> >>> job queried by 'query-block-jobs' to track its progress would be useful.
> >>>    Also, suppose I have A<- B<- C.  Does 'commit' only do one layer (C
> >>> into B), or all layers (B and C into A)?  That argues that we need an
> >>> optional parameter that says how deep to commit (committing C into B
> >>> only to repeat and commit B into A is more time-consuming than directly
> >>> committing both B and C into A to start with).  When a commit is
> >>> complete, which file is backing the device - is it still C (which
> >>> continues to diverge, but now from the point of the commit) or does qemu
> >>> pivot things to have the device now backed by B (and C can be discarded,
> >>> particularly true if changes are now going into B which invalidate C).
> >> What i find out is that 'commit' will commit changes only from C to B
> >> and qemu continues with C from the new commit point. I couldn't find a
> >> way to commit changes and go back to backing file. This should be
> >> supported by parameter and also as you mention that commit all changes
> >> through all snapshots should be supported by another parameter.
> >> The 'transaction' feature would be nice to have too.
> > Which makes it sound like we're starting to overlap with Jeff's work on
> > 'block-commit'.
> >
> > If 'block-commit' proves to be better all around at doing what we want,
> > do we even need to keep 'commit' in QMP, or would it be okay for HMP only?
> If the 'block-commit' will be better I think that we could drop the 
> 'commit' completely. And have only 'block-commit' for both QMP and HMP.

I completely agree about the QMP part, but for HMP it's a good idea to
maintain the commit command. To achieve this, we can implement hmp_commit()
in terms of block-commit.

Jeff, can you answer us here? Does block-commit supersedes the commit command
we have today?