qcow2: Implement v2 zero writes with discard if possible
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>
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@ -1797,8 +1797,15 @@ int qcow2_cluster_zeroize(BlockDriverState *bs, uint64_t offset,
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assert(QEMU_IS_ALIGNED(end_offset, s->cluster_size) ||
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assert(QEMU_IS_ALIGNED(end_offset, s->cluster_size) ||
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end_offset >= bs->total_sectors << BDRV_SECTOR_BITS);
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end_offset >= bs->total_sectors << BDRV_SECTOR_BITS);
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/* The zero flag is only supported by version 3 and newer */
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/*
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* The zero flag is only supported by version 3 and newer. However, if we
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* have no backing file, we can resort to discard in version 2.
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*/
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if (s->qcow_version < 3) {
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if (s->qcow_version < 3) {
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if (!bs->backing) {
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return qcow2_cluster_discard(bs, offset, bytes,
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QCOW2_DISCARD_REQUEST, false);
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}
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return -ENOTSUP;
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return -ENOTSUP;
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}
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}
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