δΊ 2013-7-18 13:43, Stefan Hajnoczi ει:
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