qcow2: Avoid COW during metadata preallocation
Limiting the allocation to INT_MAX bytes isn't particularly clever
because it means that the final cluster will be a partial cluster which
will be completed through a COW operation. This results in unnecessary
data read and write requests which lead to an unwanted non-sparse
filesystem block for metadata preallocation.
Align the maximum allocation size down to the cluster size to avoid this
situation.
Cc: qemu-stable@nongnu.org
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
diff --git a/block/qcow2.c b/block/qcow2.c
index 3ace3b2..dfac74c 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -2734,7 +2734,7 @@
bytes = new_length - offset;
while (bytes) {
- cur_bytes = MIN(bytes, INT_MAX);
+ cur_bytes = MIN(bytes, QEMU_ALIGN_DOWN(INT_MAX, s->cluster_size));
ret = qcow2_alloc_cluster_offset(bs, offset, &cur_bytes,
&host_offset, &meta);
if (ret < 0) {
