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qcow2: skip writing zero buffers to empty COW areas

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If COW areas of the newly allocated clusters are zeroes on the backing
image, efficient bdrv_write_zeroes(flags=BDRV_REQ_NO_FALLBACK) can be
used on the whole cluster instead of writing explicit zero buffers later
in perform_cow().

iotest 060:
write to the discarded cluster does not trigger COW anymore.
Use a backing image instead.

Signed-off-by: Anton Nefedov <anton.nefedov@virtuozzo.com>
Message-id: 20190516142749.81019-2-anton.nefedov@virtuozzo.com
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
This commit is contained in:
Anton Nefedov 2019-05-16 17:27:49 +03:00 • committed by Max Reitz
parent 330c729571
commit c8bb23cbdb
7 changed files with 106 additions and 4 deletions
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4
qapi/block-core.json
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@ -3215,6 +3215,8 @@
#
# @cor_write: a write due to copy-on-read (since 2.11)
#
# @cluster_alloc_space: an allocation of file space for a cluster (since 4.1)
#
# Since: 2.9
##
{ 'enum': 'BlkdebugEvent', 'prefix': 'BLKDBG',
@ -3233,7 +3235,7 @@
'pwritev_rmw_tail', 'pwritev_rmw_after_tail', 'pwritev',
'pwritev_zero', 'pwritev_done', 'empty_image_prepare',
'l1_shrink_write_table', 'l1_shrink_free_l2_clusters',
'cor_write'] }
'cor_write', 'cluster_alloc_space'] }
##
# @BlkdebugInjectErrorOptions: