Re: [Qemu-devel] [PATCH V9 00/14] qmp/hmp interfaces for internal snapshot info

[Wenchao Xia <xiawenc@linux.vnet.ibm.com>]

于 2013-3-13 20:40, Stefan Hajnoczi 写道:
> On Wed, Mar 13, 2013 at 06:29:18AM -0600, Eric Blake wrote:
>> On 03/13/2013 05:34 AM, Stefan Hajnoczi wrote:
>>>>
>>>> { 'type': 'BlockDeviceInfo',
>>>>    'data': { 'file': 'str', 'ro': 'bool', 'drv': 'str',
>>>>              '*backing_file': 'str', 'backing_file_depth': 'int',
>>>>              'encrypted': 'bool', 'encryption_key_missing': 'bool',
>>>>              'bps': 'int', 'bps_rd': 'int', 'bps_wr': 'int',
>>>>              'iops': 'int', 'iops_rd': 'int', 'iops_wr': 'int',
>>>>              'images': ['ImageInfo']} }}
>>>>
>>>>    In this way, new define of structure is not needed, and no break
>>>> of API I guess, but disadvantage is there will be some duplicated info
>>>> in ImageInfo and other structure in BlockInfo, such as "file,
>>>> encrypted".
>>>
>>> I'm happy with adding a ImageInfo field into BlockDeviceInfo.
>>>
>>> Why did you make it an array and what is the array's layout?
>>
>> I think the point of an array of ImageInfo is to let us return data on
>> an entire backing chain.  That is, if I have:
>>
>> image.raw <- middle.qcow2 (with internal snapshot "a") <- top.qcow2
>> (with internal snapshots "b", "c")
>>
>> then I want to see data on image.raw, as well as on snapshots a, b, and
>> c, all in one command.  For my example, 'images' would be a
>> three-element array, with element 0 describing top.qcow2 and its two
>> internal snapshots, element 1 describing middle.qcow2 and its snapshot,
>> and element 2 describing image.raw.
>>
>> However, it's also worth remembering that there has been a proposal for
>> having quorums, where a single block device can have multiple images
>> associated with the top level, and where we would then want a recursive
>> structure rather than an array for representing backing chain
>> information of a given information within the array of multiple images
>> associated with a single quorum device.
>
> The same applies for VMDK where one .vmdk can reference multiple extent
> files.
>
   I'd like to confirm: This means a block device can have multiple
images at top level, but one image can still have only one backing
image at most? If my understanding is correct,following should be the
case:

1 device hda have two active images: a.qcow2 and b.qcow2(maybe vmdk
format now, just an example)
2 a.qcow2 file's backing file will be a_base0.qcow2, never a_base0.qcow2
and a_base1.qcow2.

> This is why I asked about the array.  A dict of ImageInfos is needed,
> they can refer to each other by key.
>
> If we go with an array we're painting ourselves into a corner.
>
> If we go with a dict it's more complicated and may not be used much.
>
> I'd go for the dict but we need a policy for choosing keys (names) for
> the ImageInfo elements.  Perhaps BlockDeviceInfo['file'] is the key for
> the top-level image and then ImageInfo['backing-filename'] for the
> backing chain.
>
> Stefan
>

   Since ImageInfo already have key 'backing-filename', which stands for
the information got in this image, not from the backing image,
add a new key 'bakcing-image' for recursive linking:

{ 'type': 'ImageInfo',
   'data': {'filename': 'str', 'format': 'str', '*dirty-flag': 'bool',
            '*actual-size': 'int', 'virtual-size': 'int',
            '*cluster-size': 'int', '*encrypted': 'bool',
            '*backing-filename': 'str', '*full-backing-filename': 'str',
            '*backing-filename-format': 'str',
            '*backing-image": 'ImageInfo',
            '*snapshots': ['SnapshotInfo'] } }

   Then insert ImageInfo into BlockDeviceInfo, with key 'images', since
'file' is already used which may break compatibility if we change it.
'images' uses an array for the case when multiple images exist in
top level, not for backing chain.

{ 'type': 'BlockDeviceInfo',
   'data': { 'file': 'str', 'ro': 'bool', 'drv': 'str',
             '*backing_file': 'str', 'backing_file_depth': 'int',
             'encrypted': 'bool', 'encryption_key_missing': 'bool',
             'bps': 'int', 'bps_rd': 'int', 'bps_wr': 'int',
             'iops': 'int', 'iops_rd': 'int', 'iops_wr': 'int',
             'images': ['ImageInfo']} }}
-- 
Best Regards

Wenchao Xia