Hi all!

Here is push-backup with fleecing. What is it:

1. Make fleecing scheme

guest blk
  |
  |root
  v
copy-before-write filter  -------> temp qcow2
  |                                  |
  |file                              | backing
  V                                  |
active disk <-------------------------

2. Start backup job from temp qcow2 to final remote target (NBD or
   something)

Benefit in comparison with simple backup job: for remote final target
write operations are not very fast. And guest have to wait for
copy-before-write operations. With fleecing scheme target for
copy-before-write operations is local qcow2 file with faster access than
actual backup target. So, guest is less disturbed by copy-before-write
operations.

Based-on: <20210520142205.607501-1-vsementsov@virtuozzo.com>
  ([PATCH v2 00/33] block: publish backup-top filter)

Vladimir Sementsov-Ogievskiy (6):
  block/block-copy: use write-unchanged for fleecing scheme
  block/copy-before-write: require BLK_PERM_WRITE_UNCHANGED for fleecing
  block: share writes on backing child of fleecing node
  block: blk_root(): return non-const pointer
  block/backup: don't insert filter if no writers
  iotests/image-fleecing: test push backup with fleecing

 block/copy-before-write.h                   |   1 +
 include/sysemu/block-backend.h              |   2 +-
 block.c                                     |   3 +-
 block/backup.c                              |  55 ++++++++++-
 block/block-backend.c                       |   2 +-
 block/block-copy.c                          |  14 ++-
 block/copy-before-write.c                   |  48 ++++++++-
 tests/qemu-iotests/tests/image-fleecing     | 104 +++++++++++++++-----
 tests/qemu-iotests/tests/image-fleecing.out |  60 +++++++++++
 9 files changed, 252 insertions(+), 37 deletions(-)

-- 
2.29.2