On Fri, Jan 25, 2013 at 05:16:46PM +0800, Wenchao Xia wrote: > δΊ 2013-1-24 17:47, Stefan Hajnoczi ει: > > > >>>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
