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

[Eric Blake <eblake@redhat.com>]

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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?

-- 
Eric Blake   eblake@redhat.com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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