[Qemu-devel] [PATCH 0/7] Point-in-time snapshot exporting over NBD

[Fam Zheng <famz@redhat.com>]

This series adds for point-in-time snapshot NBD exporting based on
drive-backup. The ideas is described below and patches followed (the missing
part is item 3, which work is in progress by Ian Main who will have another
patch on it). As the work does not overlap, these series should be quite
reviewable by itself.

Background
==========

The goal of image fleecing is to provide a interface to inspect a point-in-time
snapshot of guest image data, not being interfered with guest overwrites after
it's created.  With drive-backup we already have the point-in-time snapshot
image (the target image), we only need three modifications to realize this:

 1. Give backup target an id, so we can add it to NBD server.

 2. Assign source device as backing of target, so reading the unallocated will
    be passed to source.
    As there's copy-on-write mechanism with drive-backup job, all the modified
    data after snapshot created is copied to target, the unallocated data is
    guaranteed to be unchanged, so reading from the source is correct. Note
    that this requires target format supports backing file.

 3. Adding sync mode 'none' to drive-backup, so the block job only copy changed
    data from source, which has minimal IO overhead.

Usage
=====

With above three, we can simply export a point-in-time snapshot with two QMP commands:

    drive-backup device=virtio0 format=qcow2 target=point-in-time.qcow2 target-id=pit0 sync=none
    (sync=none is not implemented for now but we can simulate with speed=1)

    nbd-server-add device=pit0 writable=no

Lifecycle
=========

Reference count for BlockDriverState is implemented to manage their lifecycles.
Device attachment, block job, backing hd, bs->file NBD and others are current
referred points of a BDS, they each call bdrv_get_ref when starting to use a
BDS and bdrv_put_ref when releasing, e.g. on hot plug/unplug, nbd_server_add,
drive-backup, etc.

I.e., when a drive-backup target bs is being exported through NBD and the backup
job finishes (or cancelled), the device is not deleted, the client can continue
to access the NBD target until nbd_server_stop. It is automatically deleted
when the last reference is released.

The target image file is not removed automatically, since we can't assume that
it's temporary. It's up to the user to remove it manually, or we need to add a
command or option to mark the target bs temporary.

Fam Zheng (7):
  block: Convert BlockDriverState.in_use to refcount
  block: use refcount to manage BlockDriverState lifecycle
  nbd: use BDS refcount
  block: simplify bdrv_drop_intermediate
  block: rename bdrv_in_use to bdrv_is_shared
  block: add target-id option to drive-backup QMP command
  block: assign backing relationship in drive-backup

 block-migration.c               |   5 +-
 block.c                         | 123 +++++++++++++++-------------------------
 block/backup.c                  |  16 +++++-
 block/blkdebug.c                |   1 +
 block/blkverify.c               |   2 +
 block/mirror.c                  |   4 +-
 block/snapshot.c                |   3 +-
 block/stream.c                  |   2 +-
 block/vvfat.c                   |   4 +-
 blockdev-nbd.c                  |   9 +--
 blockdev.c                      |  19 ++++---
 blockjob.c                      |   8 +--
 hw/block/dataplane/virtio-blk.c |   4 +-
 hw/block/xen_disk.c             |   7 +--
 include/block/block.h           |   5 +-
 include/block/block_int.h       |  18 +++++-
 nbd.c                           |   5 ++
 qapi-schema.json                |   7 ++-
 qmp-commands.hx                 |   3 +-
 19 files changed, 126 insertions(+), 119 deletions(-)

-- 
1.8.3.1