[Devel] [PATCH hci-8.1 v2 0/6] truncate unused tail of images on suspend

[Alexander Ivanov <alexander.ivanov@virtuozzo.com>]

Preallocated space and clusters at the end of an image freed by discard
are never given back, so the image file never shrinks. Truncate the
unused tail of the top image on suspend in both dm-qcow2 and dm-ploop,
so that userspace doesn't have to do it by hand.

dm-qcow2 (patches 1-2) finds the end of the last cluster with nonzero
refcount by scanning refcount blocks backward and truncates the image
to it on postsuspend.

dm-ploop needs more preparation. On "dmsetup reload" the new ploop
reads metadata in ploop_ctr(), while IO still goes through the old one,
so after resume it may work with outdated BAT and file sizes. Patches
3-4 fix a raw delta handling bug and factor out ploop_read_delta(),
patch 5 reloads all the metadata in ploop_preresume(), and patch 6
truncates the top delta to the end of the last used cluster (BAT area,
mapped cluster or format extension) on postsuspend.

The corresponding ploop userspace changes:
https://bitbucket.org/openvz/ploop.ovz/pull-requests/486

https://virtuozzo.atlassian.net/browse/VSTOR-148171

v2:
- Add dm-ploop support (patches 3-6), as requested in v1 review.
- dm-qcow2: move get_md_page_sync() to a separate patch as
  qcow2_md_page_read_sync() and use it in qcow2_parse_header() as well.
- dm-qcow2: start the refcount scan below file_preallocated_area_start
  instead of the end of the file, and truncate to the used end directly
  instead of min(file_preallocated_area_start, used end).

Alexander Ivanov (6):
  dm-qcow2: add qcow2_md_page_read_sync() helper
  dm-qcow2: truncate unused tail of image on suspend
  dm-ploop: set delta is_raw before applying its mappings
  dm-ploop: factor out ploop_read_delta() from ploop_add_delta()
  dm-ploop: reload metadata on resume
  dm-ploop: truncate unused tail of top delta on suspend

 drivers/md/dm-ploop-bat.c    | 229 +++++++++++++++++++++++++++++++++--
 drivers/md/dm-ploop-map.c    |  26 +++-
 drivers/md/dm-ploop-target.c | 100 +++++++++++----
 drivers/md/dm-ploop.h        |   8 +-
 drivers/md/dm-qcow2-map.c    |  53 ++++++++
 drivers/md/dm-qcow2-target.c |  48 +++++++-
 drivers/md/dm-qcow2.h        |   3 +
 7 files changed, 421 insertions(+), 46 deletions(-)

-- 
2.53.0

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