[Qemu-devel] [RFC PATCH v3 0/2] Point-in-time snapshot exporting over NBD

[Fam Zheng <famz@redhat.com>]

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