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

From: Wenchao Xia
Subject: Re: [Qemu-devel] [PATCH 0/4] export internal snapshot by qemu-nbd
Date: Mon, 22 Jul 2013 11:26:49 +0800
User-agent: Mozilla/5.0 (Windows NT 5.1; rv:17.0) Gecko/20130620 Thunderbird/17.0.7

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?


  Rechecked VMDK's doc, guess it is using backing chain. Still, It helps
if internal snapshot can do similar things.

Best Regards

Wenchao Xia

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