The compression filter driver is introduced as suggested by Max.
A sample usage of the filter can be found in the test #214.
Now, multiple clusters can be written compressed.
It is useful for the backup job.

v9: The filter functions unrelated to the 'file' child were removed.
    The check for compression support in bs->file is checked in the
    compress_open().

  Discussed in the email thread with the message ID
  <1574056144-625164-1-git-send-email-andrey.shinkevich@virtuozzo.com>

Andrey Shinkevich (3):
  block: introduce compress filter driver
  qcow2: Allow writing compressed data of multiple clusters
  tests/qemu-iotests: add case to write compressed data of multiple
    clusters

 block/Makefile.objs        |   1 +
 block/filter-compress.c    | 190 +++++++++++++++++++++++++++++++++++++++++++++
 block/qcow2.c              | 102 +++++++++++++++++-------
 qapi/block-core.json       |  10 ++-
 tests/qemu-iotests/214     |  43 ++++++++++
 tests/qemu-iotests/214.out |  14 ++++
 6 files changed, 329 insertions(+), 31 deletions(-)
 create mode 100644 block/filter-compress.c

-- 
1.8.3.1