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qcow2: Set zero flag for discarded clusters

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Instead of making the backing file contents visible again after a discard
request, set the zero flag if possible (i.e. on version >= 3).

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
This commit is contained in:
Kevin Wolf 2014-02-08 14:38:33 +01:00
parent 3e890c77cf
commit a71835a0cc
2 changed files with 34 additions and 6 deletions
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22
block/qcow2-cluster.c
View file

@ -1367,13 +1367,31 @@ static int discard_single_l2(BlockDriverState *bs, uint64_t offset,
uint64_t old_offset;
old_offset = be64_to_cpu(l2_table[l2_index + i]);
if ((old_offset & L2E_OFFSET_MASK) == 0) {
/*
* Make sure that a discarded area reads back as zeroes for v3 images
* (we cannot do it for v2 without actually writing a zero-filled
* buffer). We can skip the operation if the cluster is already marked
* as zero, or if it's unallocated and we don't have a backing file.
*
* TODO We might want to use bdrv_get_block_status(bs) here, but we're
* holding s->lock, so that doesn't work today.
*/
if (old_offset & QCOW_OFLAG_ZERO) {
continue;
}
if ((old_offset & L2E_OFFSET_MASK) == 0 && !bs->backing_hd) {
continue;
}
/* First remove L2 entries */
qcow2_cache_entry_mark_dirty(s->l2_table_cache, l2_table);
l2_table[l2_index + i] = cpu_to_be64(0);
if (s->qcow_version >= 3) {
l2_table[l2_index + i] = cpu_to_be64(QCOW_OFLAG_ZERO);
} else {
l2_table[l2_index + i] = cpu_to_be64(0);
}
/* Then decrease the refcount */
qcow2_free_any_clusters(bs, old_offset, 1, type);