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qcow2: Implement v2 zero writes with discard if possible

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qcow2 version 2 images don't support the zero flag for clusters, so for
write_zeroes requests, we return -ENOTSUP and get explicit zero buffer
writes. If the image doesn't have a backing file, we can do better: Just
discard the respective clusters.

This is relevant for 'qemu-img convert -O qcow2 -n', where qemu-img has
to assume that the existing target image may contain any data, so it has
to write zeroes. Without this patch, this results in a fully allocated
target image, even if the source image was empty.

Reported-by: Nir Soffer <nsoffer@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20200721135520.72355-2-kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
This commit is contained in:
Kevin Wolf 2020-07-21 15:55:19 +02:00
parent bae127d4dc
commit 61b3043965
1 changed files with 8 additions and 1 deletions
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9
block/qcow2-cluster.c
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@ -1797,8 +1797,15 @@ int qcow2_cluster_zeroize(BlockDriverState *bs, uint64_t offset,
assert(QEMU_IS_ALIGNED(end_offset, s->cluster_size) ||
end_offset >= bs->total_sectors << BDRV_SECTOR_BITS);
/* The zero flag is only supported by version 3 and newer */
/*
* The zero flag is only supported by version 3 and newer. However, if we
* have no backing file, we can resort to discard in version 2.
*/
if (s->qcow_version < 3) {
if (!bs->backing) {
return qcow2_cluster_discard(bs, offset, bytes,
QCOW2_DISCARD_REQUEST, false);
}
return -ENOTSUP;
}