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qcow2: Allow "full" discard

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Normally, discarded sectors should read back as zero. However, there are
cases in which a sector (or rather cluster) should be discarded as if
they were never written in the first place, that is, reading them should
fall through to the backing file again.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Message-id: 1414159063-25977-2-git-send-email-mreitz@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
This commit is contained in:
Max Reitz 2014-10-24 15:57:30 +02:00 • committed by Stefan Hajnoczi
parent 70a5ff6bdd
commit 808c4b6f30
4 changed files with 20 additions and 13 deletions
Download patch file Download diff file Expand all files Collapse all files

27
block/qcow2-cluster.c
View file

@ -1414,7 +1414,7 @@ int qcow2_decompress_cluster(BlockDriverState *bs, uint64_t cluster_offset)
* clusters.
*/
static int discard_single_l2(BlockDriverState *bs, uint64_t offset,
unsigned int nb_clusters, enum qcow2_discard_type type)
unsigned int nb_clusters, enum qcow2_discard_type type, bool full_discard)
{
BDRVQcowState *s = bs->opaque;
uint64_t *l2_table;
@ -1436,23 +1436,30 @@ static int discard_single_l2(BlockDriverState *bs, uint64_t offset,
old_l2_entry = be64_to_cpu(l2_table[l2_index + i]);
/*
* 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.
* If full_discard is false, 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 full_discard is true, the sector should not read back as zeroes,
* but rather fall through to the backing file.
*/
switch (qcow2_get_cluster_type(old_l2_entry)) {
case QCOW2_CLUSTER_UNALLOCATED:
if (!bs->backing_hd) {
if (full_discard || !bs->backing_hd) {
continue;
}
break;
case QCOW2_CLUSTER_ZERO:
continue;
if (!full_discard) {
continue;
}
break;
case QCOW2_CLUSTER_NORMAL:
case QCOW2_CLUSTER_COMPRESSED:
@ -1464,7 +1471,7 @@ static int discard_single_l2(BlockDriverState *bs, uint64_t offset,
/* First remove L2 entries */
qcow2_cache_entry_mark_dirty(s->l2_table_cache, l2_table);
if (s->qcow_version >= 3) {
if (!full_discard && 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);
@ -1483,7 +1490,7 @@ static int discard_single_l2(BlockDriverState *bs, uint64_t offset,
}
int qcow2_discard_clusters(BlockDriverState *bs, uint64_t offset,
int nb_sectors, enum qcow2_discard_type type)
int nb_sectors, enum qcow2_discard_type type, bool full_discard)
{
BDRVQcowState *s = bs->opaque;
uint64_t end_offset;
@ -1506,7 +1513,7 @@ int qcow2_discard_clusters(BlockDriverState *bs, uint64_t offset,
/* Each L2 table is handled by its own loop iteration */
while (nb_clusters > 0) {
ret = discard_single_l2(bs, offset, nb_clusters, type);
ret = discard_single_l2(bs, offset, nb_clusters, type, full_discard);
if (ret < 0) {
goto fail;
}