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.

v6: The new approach to write compressed data was applied. The patch
    v5 4/4 with the block-stream compress test case was removed from
    the series.
    Discussed in the email thread with the message ID
    <1571603828-185910-1-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    | 212 +++++++++++++++++++++++++++++++++++++++++++++
 block/qcow2.c              | 102 ++++++++++++++++------
 qapi/block-core.json       |  10 ++-
 tests/qemu-iotests/214     |  43 +++++++++
 tests/qemu-iotests/214.out |  14 +++
 6 files changed, 351 insertions(+), 31 deletions(-)
 create mode 100644 block/filter-compress.c

-- 
1.8.3.1