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authorEric Blake <eblake@redhat.com>2017-05-06 19:05:47 -0500
committerMax Reitz <mreitz@redhat.com>2017-05-11 14:28:07 +0200
commit06cc5e2b2d01cb778c966e1b4135062556b3b054 (patch)
treed8992a233e2b280340332858de85ae09245ae025 /block/qcow2-cluster.c
parentfdfab37dfeffefbd4533b4158055c9b82d7c3e69 (diff)
qcow2: Optimize zero_single_l2() to minimize L2 churn
Similar to discard_single_l2(), we should try to avoid dirtying the L2 cache when the cluster we are changing already has the right characteristics. Note that by the time we get to zero_single_l2(), BDRV_REQ_MAY_UNMAP is a requirement to unallocate a cluster (this is because the block layer clears that flag if discard.* flags during open requested that we never punch holes - see the conversation around commit 170f4b2e, https://lists.gnu.org/archive/html/qemu-devel/2016-09/msg07306.html). Therefore, this patch can only reuse a zero cluster as-is if either unmapping is not requested, or if the zero cluster was not associated with an allocation. Technically, there are some cases where an unallocated cluster already reads as all zeroes (namely, when there is no backing file [easy: check bs->backing], or when the backing file also reads as zeroes [harder: we can't check bdrv_get_block_status since we are already holding the lock]), where the guest would not immediately see a difference if we left that cluster unallocated. But if the user did not request unmapping, leaving an unallocated cluster is wrong; and even if the user DID request unmapping, keeping a cluster unallocated risks a subtle semantic change of guest-visible contents if a backing file is later added, and it is not worth auditing whether all internal uses such as mirror properly avoid an unmap request. Thus, this patch is intentionally limited to just clusters that are already marked as zero. Signed-off-by: Eric Blake <eblake@redhat.com> Reviewed-by: Max Reitz <mreitz@redhat.com> Message-id: 20170507000552.20847-8-eblake@redhat.com Signed-off-by: Max Reitz <mreitz@redhat.com>
Diffstat (limited to 'block/qcow2-cluster.c')
-rw-r--r--block/qcow2-cluster.c15
1 files changed, 13 insertions, 2 deletions
diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c
index 558c23957f..e2c5759d94 100644
--- a/block/qcow2-cluster.c
+++ b/block/qcow2-cluster.c
@@ -1601,6 +1601,7 @@ static int zero_single_l2(BlockDriverState *bs, uint64_t offset,
int l2_index;
int ret;
int i;
+ bool unmap = !!(flags & BDRV_REQ_MAY_UNMAP);
ret = get_cluster_table(bs, offset, &l2_table, &l2_index);
if (ret < 0) {
@@ -1613,12 +1614,22 @@ static int zero_single_l2(BlockDriverState *bs, uint64_t offset,
for (i = 0; i < nb_clusters; i++) {
uint64_t old_offset;
+ QCow2ClusterType cluster_type;
old_offset = be64_to_cpu(l2_table[l2_index + i]);
- /* Update L2 entries */
+ /*
+ * Minimize L2 changes if the cluster already reads back as
+ * zeroes with correct allocation.
+ */
+ cluster_type = qcow2_get_cluster_type(old_offset);
+ if (cluster_type == QCOW2_CLUSTER_ZERO_PLAIN ||
+ (cluster_type == QCOW2_CLUSTER_ZERO_ALLOC && !unmap)) {
+ continue;
+ }
+
qcow2_cache_entry_mark_dirty(bs, s->l2_table_cache, l2_table);
- if (old_offset & QCOW_OFLAG_COMPRESSED || flags & BDRV_REQ_MAY_UNMAP) {
+ if (cluster_type == QCOW2_CLUSTER_COMPRESSED || unmap) {
l2_table[l2_index + i] = cpu_to_be64(QCOW_OFLAG_ZERO);
qcow2_free_any_clusters(bs, old_offset, 1, QCOW2_DISCARD_REQUEST);
} else {