Re: [Qemu-devel] [RFC] qemu snapshot enchancement

[Stefan Hajnoczi]

On Fri, Jan 25, 2013 at 05:16:46PM +0800, Wenchao Xia wrote:
> 于 2013-1-24 17:47, Stefan Hajnoczi 写道:
> >
> >>>Case 3:
> >>>
> >>>  * What does "blank data" mean?  Besides that the use case
> >>>    makes sense.
> >>>
> >>   Will remove the words.
> >>
> >>>  * When discussing this use case in the past it was suggested that the
> >>>    guest doesn't need to be paused during the LVM snapshot.  Instead the
> >>>    QEMU block layer might be able to queue I/O requests, allowing the
> >>>    guest to run.
> >>>
> >>   That is a good idea, but seems need more work(event, block layer...),
> >>hope it can be added as an enchancement of this case. Now let the
> >>dedicated storage software/hardware take the job by pausing for a while
> >>(<200ms?)
> >
> >Yes, allowing the guest to continue but queuing I/O will require extra
> >block layer work and maybe a QMP command.  There is a also a risk: if
> >the snapshot takes too long to complete, the guest may notice that its
> >I/O request are taking a long time.  It may decide that they have timed
> >out and report an error to the application or in the message logs.
> >
> >In the beginning it's easier to pause the VM but let's keep queuing I/O
> >in mind so it can be added later, if necessary.
> >
>   Yep, the code should leave a room for queuing.
> 
>   I have updated the wiki, which added the step details of the cases
> . Cases 3 are fixed, which have best performance in qemu management
> type. But I am not sure if it is workable in theory to export base
> data of an qcow2 image having internal snapshot, what do you think
> of it?

Yes, it is theoretically possible to access snapshot data while the
guest is running.

Open the qcow2 read-only and use bdrv_snapshot_load_tmp() to activate
the snapshot without modifying the qcow2 file on disk.  This is pretty
easy to implement today by adding options to qemu-nbd or the in-process
NBD server QMP commands.

Stefan