[Qemu-devel] [RFC PATCH 00/17] Improvements around discard and write zeroes

[Paolo Bonzini]

This series reworks write_zeroes and discard support in QEMU, with
the aim of enabling discard support in guests.  The problem here is
that so far the semantics of discard were left to the block drivers.
Discard could only be enabled as long as all storage had the same
characteristics, including all of them supporting it and all of them
having the same discard granularity.  Hence, nobody really bothered to
enable it.  (This is good, because we have fewer backwards-compatibility
issues).

This series fixes this by specifying the semantics of discard at the block
layer level: from now on, discard will always have sector granularity
and write zeroes to the devices.  Given this choice, the equivalent
of Linux discard_zeroes_data can also be easily implemented.

Patches 1-4 add information on discard to bdrv_get_info, and implement
discard for QED.

Patches 5-8 replace the write_zeroes operation with discard.

Patches 9-12 turn discard on in the device models.

Patches 13-15 add discard support for files and enable hole-punching
in other filesystems than XFS.

To complete thin provisioning support, patches 16 and 17 implement
is_allocated for raw using the brand new SEEK_HOLE/SEEK_DATA
interface.  With these patches and with the kernel patch at
http://patchwork.xfs.org/patch/3264/ it should be possible to do efficient
streaming with raw on XFS.

Various bits could be separated and committed in parts; comments
are welcome.

Paolo Bonzini (17):
  qemu-iotests: add a simple test for write_zeroes
  qed: make write-zeroes bounce buffer smaller than a single cluster
  block: add discard properties to BlockDriverInfo
  qed: implement bdrv_aio_discard
  block: pass around qiov for write_zeroes operation
  block: use bdrv_{co,aio}_discard for write_zeroes operations
  block: make high level discard operation always zero
  block: kill the write zeroes operation
  ide: issue discard asynchronously but serialize the pieces
  ide/scsi: add discard_zeroes_data property
  ide/scsi: prepare for flipping the discard defaults
  ide/scsi: turn on discard
  block: fallback from discard to writes
  block: support FALLOC_FL_PUNCH_HOLE trimming
  raw: add get_info
  qemu-io: fix the alloc command
  raw: implement is_allocated

 block.c                    |   62 +++++++------
 block.h                    |   30 ++++---
 block/qcow2.c              |    2 +
 block/qed.c                |  126 ++++++++++++-------------
 block/raw-posix.c          |  100 +++++++++++++++++++-
 block/raw.c                |   14 +++
 block_int.h                |    8 --
 hw/ide/core.c              |   81 +++++++++++-----
 hw/ide/qdev.c              |    6 +-
 hw/pc_piix.c               |  224 ++++++++++++++++++++++++++++++++++++++++++++
 hw/scsi-disk.c             |   17 +++-
 qemu-io.c                  |   61 ++----------
 tests/qemu-iotests/031     |   73 ++++++++++++++
 tests/qemu-iotests/031.out |   29 ++++++
 tests/qemu-iotests/group   |    1 +
 15 files changed, 635 insertions(+), 199 deletions(-)
 create mode 100755 tests/qemu-iotests/031
 create mode 100644 tests/qemu-iotests/031.out

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1.7.7.6