Re: [Qemu-devel] [PATCH v2 00/15] qcow2: space preallocation and COW improvements

[John Snow <jsnow@redhat.com>]

Missing qemu-block@, CCing.

On 06/01/2017 11:14 AM, Anton Nefedov wrote:
> Changes in v2:
>   - introduce new BDRV flag for write_zeroes()
>       instead of using driver callback directly.
>     Skipped introducing new functions like bdrv_co_pallocate() for now:
>       1. it seems ok to keep calling this write_zeroes() as zeroes
>       are expected;
>       2. most of the code can be reused now anyway, so changes to
>       write_zeroes() path are not significant
>       3. write_zeroes() alignment and max-request limits can also be reused
> 
>     As a possible alternative we can have bdrv_co_pallocate() which can
>     switch to pwrite_zeroes(,flags|=BDRV_REQ_ALLOCATE) early.
> 
> ========
> 
> This pull request is to address 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. This can be avoided
>     and the whole clusters are preallocated and zeroed in a single
>     efficient write_zeroes() operation, also providing better host file
>     continuity
> 
>   2. moreover, efficient write_zeroes() operation can be used to preallocate
>     space megabytes ahead which gives noticeable improvement on some storage
>     types (e.g. distributed storages where space allocation operation is
>     expensive)
> 
>   3. preallocating/zeroing the clusters in advance makes possible to enable
>     simultaneous writes to the same unallocated cluster, which is beneficial
>     for parallel sequential write operations which are not cluster-aligned
> 
> Performance test results are added to commit messages (see patch 3, 12)
> 
> Anton Nefedov (11):
>   block: introduce BDRV_REQ_ALLOCATE flag
>   file-posix: support BDRV_REQ_ALLOCATE
>   blkdebug: support BDRV_REQ_ALLOCATE
>   qcow2: do not COW the empty areas
>   qcow2: set inactive flag
>   qcow2: handle_prealloc(): find out if area zeroed by earlier
>     preallocation
>   qcow2: fix misleading comment about L2 linking
>   qcow2-cluster: slightly refactor handle_dependencies()
>   qcow2-cluster: make handle_dependencies() logic easier to follow
>   qcow2: allow concurrent unaligned writes to the same clusters
>   iotest 046: test simultaneous cluster write error case
> 
> Denis V. Lunev (3):
>   qcow2: alloc space for COW in one chunk
>   qcow2: preallocation at image expand
>   qcow2: truncate preallocated space
> 
> Pavel Butsykin (1):
>   qcow2: check space leak at the end of the image
> 
>  block/blkdebug.c                   |   3 +-
>  block/file-posix.c                 |   9 +-
>  block/io.c                         |  19 ++-
>  block/qcow2-cache.c                |   3 +
>  block/qcow2-cluster.c              | 218 +++++++++++++++++++++++------
>  block/qcow2-refcount.c             |  21 +++
>  block/qcow2.c                      | 273 ++++++++++++++++++++++++++++++++++++-
>  block/qcow2.h                      |  26 ++++
>  block/trace-events                 |   1 +
>  include/block/block.h              |   6 +-
>  tests/qemu-iotests/026.out         | 104 ++++++++++----
>  tests/qemu-iotests/026.out.nocache | 104 ++++++++++----
>  tests/qemu-iotests/029.out         |   5 +-
>  tests/qemu-iotests/046             |  38 +++++-
>  tests/qemu-iotests/046.out         |  23 ++++
>  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 +-
>  23 files changed, 789 insertions(+), 112 deletions(-)
>