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[Qemu-devel] [PATCH v5 00/15] qcow2: space preallocation and COW improve

From: Anton Nefedov
Subject: [Qemu-devel] [PATCH v5 00/15] qcow2: space preallocation and COW improvements
Date: Wed, 1 Nov 2017 18:43:53 +0300

v5: rebased, patches slightly reordered;
    cluster allocation (to avoid writing zero-buffers)
    now goes before an optional ahead preallocation

v4: http://lists.nongnu.org/archive/html/qemu-devel/2017-08/msg00109.html

This pull request is to improve a few performance problems of qcow2 format:

  1. non cluster-aligned write requests (to unallocated clusters) explicitly
     pad data with zeroes if there is no backing data.
     Resulting increase in ops number and potential cluster fragmentation
     (on the host file) is already solved by:
       ee22a9d qcow2: Merge the writing of the COW regions with the guest data
     However, in case of zero COW regions, that can be avoided at all
     but the whole clusters are preallocated and zeroed in a single
     efficient write_zeroes() operation

  2. moreover, efficient write_zeroes() operation can be used to preallocate
     space megabytes (*configurable number) ahead which gives noticeable
     improvement on some storage types (e.g. distributed storage)
     where the space allocation operation might be expensive)

  3. this will also allow to enable simultaneous writes to the same unallocated
     cluster after the space has been allocated & zeroed but before
     the first data is written and the cluster is linked to L2.
     (Not included in this patchset).

Efficient write_zeroes usually implies that the blocks are not actually
written to but only reserved and marked as zeroed by the storage.
In this patchset, file-posix driver is marked as supporting this operation
if it supports (/configured to support) fallocate() operation.

Existing bdrv_write_zeroes() falls back to writing zero buffers if
write_zeroes is not supported by the driver.
A new flag (BDRV_REQ_ALLOCATE) is introduced to avoid that but return ENOTSUP.
Such allocate requests are also implemented to possibly overlap with the
other requests. No wait is performed but an error returned in such case as well.
So the operation should be considered advisory and a fallback scenario still
handled by the caller (in this case, qcow2 driver).

Anton Nefedov (12):
  mirror: inherit supported write/zero flags
  blkverify: set supported write/zero flags
  block: introduce BDRV_REQ_ALLOCATE flag
  block: treat BDRV_REQ_ALLOCATE as serialising
  file-posix: support BDRV_REQ_ALLOCATE
  block: support BDRV_REQ_ALLOCATE in passthrough drivers
  qcow2: move is_zero() up
  qcow2: skip writing zero buffers to empty COW areas
  qcow2: set inactive flag
  qcow2: do not zero out clusters if already preallocated
  iotest 198: test BDRV_REQ_ALLOCATE
  iotest 134: test cluster-misaligned encrypted write

Denis V. Lunev (2):
  qcow2: preallocation at image expand
  qcow2: truncate preallocated space

Pavel Butsykin (1):
  qcow2: check space leak at the end of the image

 block/qcow2.h                      |  18 ++++
 include/block/block.h              |   6 +-
 include/block/block_int.h          |   2 +-
 block/blkdebug.c                   |   3 +-
 block/blkverify.c                  |   9 ++
 block/file-posix.c                 |   8 ++
 block/io.c                         |  47 +++++++--
 block/mirror.c                     |   5 +
 block/qcow2-cluster.c              |  14 ++-
 block/qcow2-refcount.c             |   7 ++
 block/qcow2.c                      | 202 +++++++++++++++++++++++++++++++++----
 block/raw-format.c                 |   3 +-
 block/trace-events                 |   1 +
 qemu-options.hx                    |   4 +
 tests/qemu-iotests/026.out         |  94 +++++++++++++----
 tests/qemu-iotests/026.out.nocache |  94 +++++++++++++----
 tests/qemu-iotests/029.out         |   5 +-
 tests/qemu-iotests/060             |   2 +-
 tests/qemu-iotests/060.out         |  13 ++-
 tests/qemu-iotests/061.out         |   5 +-
 tests/qemu-iotests/066             |   2 +-
 tests/qemu-iotests/066.out         |   9 +-
 tests/qemu-iotests/098.out         |   7 +-
 tests/qemu-iotests/108.out         |   5 +-
 tests/qemu-iotests/112.out         |   5 +-
 tests/qemu-iotests/134             |   9 ++
 tests/qemu-iotests/134.out         |  10 ++
 tests/qemu-iotests/198             | 146 +++++++++++++++++++++++++++
 tests/qemu-iotests/198.out         |  50 +++++++++
 tests/qemu-iotests/group           |   1 +
 30 files changed, 693 insertions(+), 93 deletions(-)
 create mode 100755 tests/qemu-iotests/198
 create mode 100644 tests/qemu-iotests/198.out


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