Re: [Qemu-devel] [PATCH V7 11/14] qmp: add interface query-snapshots

[Eric Blake]

On 02/27/2013 07:14 PM, Wenchao Xia wrote:
> 于 2013-2-28 0:08, Markus Armbruster 写道:
>> Wenchao Xia <address@hidden> writes:
>>
>>>    This interface now return valid internal snapshots for whole vm.
>>>    Note that filter use bdrv_can_read_snapshot() instead of
>>> bdrv_can_snapshot(), which should be the correct behavior in information
>>> retrieving funtion.
>>
>> This is just like query-images, except it's for the system as a whole
>> rather than individual block devices.  Correct?
>>
>   Yes, just for snapshot info for whole vm.

Another way of looking at it: if a guest has two images, a.qcow and
b.qcow, where a.qcow includes snapshot id '1' named 'bar', and id '2'
named 'baz'; and where b.qcow includes snapshot id '1' named 'foo', and
id '2' named 'bar', then querying individual block devices will see both
internal snapshots of each device, but querying the system as a whole
will see only a single snapshot named 'bar' and with no reliable id.
This is because the 'loadvm' HMP command (or whatever equivalent we
finally ever make in QMP) needs to act on all devices at once; reverting
to id '1' or name 'foo' doesn't make much sense, because those are not
consistent.  A management app might be able to piece things together
through a more complicated disk-at-a-time transaction, at a lower level
than what 'loadvm' does.  But this command seems useful for stating what
snapshots are consistent enough for 'loadvm' to have a chance of succeeding.

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

signature.asc
Description: OpenPGP digital signature