Re: [Qemu-devel] [PATCH 0/4] export internal snapshot by qemu-nbd

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

于 2013-7-22 10:10, Fam Zheng 写道:
> On Fri, 07/19 17:03, Wenchao Xia wrote:
>> 于 2013-7-18 13:43, Stefan Hajnoczi 写道:
>>> On Wed, Jul 17, 2013 at 10:03:51PM +0800, Wenchao Xia wrote:
>>>> This series allow user to read internal snapshot's contents without qemu-img
>>>> convert. Another purpose is that, when qemu is online and have taken an
>>>> internal snapshot, let user invoke qemu-nbd to do any thing on it except write.
>>>
>>> I agree with Eric and Kevin that we cannot access image files while QEMU
>>> has them open.
>>>
>>> A bit more detail about using the run-time NBD server to do this safely:
>>>
>>> Internal snapshots are not first-class block layer objects today.  They
>>> are not BlockDriverStates, instead you must access their data through
>>> bdrv_snapshot_goto() or bdrv_snapshot_load_tmp().
>>>
>>    Also cc to Fam Zheng an Kevin:
>>    I think BlockDriverStates modification, is based on a more basic
>> question: does qcow2's data structure on disk, allow multiple snapshot
>> reader, while one active writer, without lock? I think the modification
>> showed above, already said "yes" to this question. If yes, just
>> create a new BlockDriverStates, we should say yes in spec.
>>    I wonder how vmdk allow direct snapshot access by VixDiskLib 1.1,
>> without network expense on host, it should be the format said support
>> of it.
>>
> Is the VMDK snapshot internal? Which subformat of VMDK do you mean here?
>
> Thanks.
>
   Rechecked VMDK's doc, guess it is using backing chain. Still, It helps
if internal snapshot can do similar things.

-- 
Best Regards

Wenchao Xia