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

[Pavel Hrdina <phrdina@redhat.com>]

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.

Pavel