[Qemu-devel] [PATCH v4 0/7] Drop in_use from BlockDriverState and enable point-in-time snapshot exporting over NBD

[Fam Zheng]

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

v4: Dropping RFC, this series tries to address the crashing cases with an added
    safety mechanics.
    
    In the first half of series, replace the in_use flag with an operation
    blocker list, and block all operations on named backing hd:

    [01/07] qapi: Add BlockOperationType enum
    [02/07] block: Introduce op_blockers to BlockDriverState
    [03/07] block: Replace in_use with operation blocker
    [04/07] block: Add checks of blocker in block operations

    The second half enables point in time snapshot over NBD, with fixes from
    last revision:

    [05/07] block: Parse "backing" option to reference existing BDS
            Fix NULL dereference if device not found.
    [06/07] qmp: add command 'blockdev-backup'
            Moved some checks into backup_run. (Paolo)
    [07/07] block: Allow backup on referenced named BlockDriverState
            New. Removes one specific blocker on backing referenced BDS, so we
            can start a backup job on it.

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 (7):
  qapi: Add BlockOperationType enum
  block: Introduce op_blockers to BlockDriverState
  block: Replace in_use with operation blocker
  block: Add checks of blocker in block operations
  block: Parse "backing" option to reference existing BDS
  qmp: add command 'blockdev-backup'
  block: Allow backup on referenced named BlockDriverState

 block-migration.c               |   8 ++-
 block.c                         | 124 ++++++++++++++++++++++++++++++++++------
 block/backup.c                  |  22 +++++++
 blockdev-nbd.c                  |   4 ++
 blockdev.c                      | 100 ++++++++++++++++++++++++++++----
 blockjob.c                      |  12 ++--
 hw/block/dataplane/virtio-blk.c |  16 ++++--
 include/block/block.h           |   8 ++-
 include/block/block_int.h       |   9 ++-
 include/block/blockjob.h        |   3 +
 qapi-schema.json                |  74 ++++++++++++++++++++++++
 qmp-commands.hx                 |  46 +++++++++++++++
 12 files changed, 384 insertions(+), 42 deletions(-)

-- 
1.8.4.2