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
v3: Base on blockdev-add.
The syntax blockdev-add backing=<id> is new: This will make referenced BDS
in the middle of a backing chain, which has significant effects over all
existing block operations in QMP. It needs to be reviewed and tested
carefully.
I'm listing the commands here that can take a device id as parameter (but
may not be complete).
These commands do not mutate the backing chain (not directly, at least) and
should be safe:
block_passwd
block_set_io_throttle
block-job-set-speed
block-job-cancel
block-job-pause
block-job-resume
block-job-complete
drive-backup
blockdev-snapshot-sync
blockdev-snapshot-internal-sync
blockdev-snapshot-delete-internal-sync: These should be safe.
nbd-server-add
These can mutates the chain (removing, closing or swapping BDS), need more
work to convert them to safe operations with a BDS in the middle.
device_del
eject: it does bdrv_close the device.
change: internally calls eject.
block-commit: it deletes intermediate BDSes, which may include other
named BDS.
block-stream:
drive-mirror: it swaps active with target when complete.
Resizing a middle BDS need to be reviewed too:
block_resize: TBD.
Adding and backing HD referencing will put other BDS in middle, but should
not immediately break things:
blockdev-add
Fam Zheng (2):
block: parse "backing" option to reference existing BDS
qmp: add command 'blockdev-backup'
block.c | 27 +++++++++++++++++++-----
blockdev.c | 63 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++
qapi-schema.json | 49 +++++++++++++++++++++++++++++++++++++++++++
qmp-commands.hx | 46 +++++++++++++++++++++++++++++++++++++++++
4 files changed, 180 insertions(+), 5 deletions(-)
--
1.8.3.1
