On Fri, 11/22 17:58, Stefan Hajnoczi wrote: > On Fri, Nov 22, 2013 at 01:24:47PM +0800, Fam Zheng wrote: > > This series adds for point-in-time snapshot NBD exporting based on > > blockdev-backup (variant of drive-backup with existing device as target). > > > > We get a thin point-in-time snapshot by COW mechanism of drive-backup, and > > export it through built in NBD server. The steps are as below: > > > > 1. (SHELL) qemu-img create -f qcow2 BACKUP.qcow2 <source size here> > > > > (Alternatively we can use -o backing_file=RUNNING-VM.img to omit > > explicitly providing the size by ourselves, but it's risky because > > RUNNING-VM.qcow2 is used r/w by guest. Whether or not setting backing > > file in the image file doesn't matter, as we are going to override the > > backing hd in the next step) > > > > 2. (QMP) blockdev-add backing=source-drive file.driver=file > > file.filename=BACKUP.qcow2 id=target0 if=none driver=qcow2 > > > > (where ide0-hd0 is the running BlockDriverState name for > > RUNNING-VM.img. This patch implements "backing=" option to override > > backing_hd for added drive) > > > > 3. (QMP) blockdev-backup device=source-drive sync=none target=target0 > > > > (this is the QMP command introduced by this series, which use a named > > device as target of drive-backup) > > > > 4. (QMP) nbd-server-add device=target0 > > > > When image fleecing done: > > > > 1. (QMP) block-job-complete device=ide0-hd0 > > > > 2. (HMP) drive_del target0 > > > > 3. (SHELL) rm BACKUP.qcow2 > > Interesting implementation, it looks pretty good. I'll need to review it a > second time to track all the operation block/unblocks. It wasn't immediately > clear to me whether these patches will restrict something that used to work. >
Good question, I asked myself too. :) In some point in the middle of the series it should be theoretically the same as before. But I did add some more blocker checks, E.g. NBD is blocked if there's a block job, but starting nbd add doesn't add blocker. So we can nbd_server_add then start block job, but not in the other order. This is kind of weird. I will also look back again. I think another option is a compatibility matrix, to simplify the blocker interface to bdrv_op_try_start(bs, op) + bdrv_op_end(bs, op): We get less flexibility, but don't need dynanically allocated blocker from caller. The advantage is that it's more centralized logic, so it's easy to manage. Fam
