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

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

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.




> The problem with these functions is that they use the BlockDriverState.
> So the guest cannot write to the main image while an internal snapshot
> is loaded.
>
> This can be solved by introducing a function that creates a read-only
> internal snapshot BlockDriverState which is partially independent of the
> main image BlockDriverState that the guest is accessing.
>
> The challenge is implementing this cleanly:
>
> 1. The lifecycle of the main image BlockDriverState and the read-only
>     internal snapshot BlockDriverState is related.  What happens when the
>     main image BDS is deleted but the snapshot BDS is still alive?  What
>     happens when bdrv_snapshot_delete() is called while the BDS snapshot
>     exists?
>
> 2. At what level should resources like the bs->file and metadata caches
>     (L1/L2/refcount) be shared?  If they are shared it's easy to keep
>     them consistent but makes lifecycle handling harder.
>
> Stefan
>


-- 
Best Regards

Wenchao Xia