[PULL,09/27] qcow2: Avoid COW during metadata preallocation
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Message ID 20190430154244.30083-10-kwolf@redhat.com
State New
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Series
  • [PULL,01/27] tests/qemu-iotests: Fix output of qemu-io related tests
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Commit Message

Kevin Wolf April 30, 2019, 3:42 p.m. UTC
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>
---
 block/qcow2.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Patch
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diff --git a/block/qcow2.c b/block/qcow2.c
index 3ace3b2209..dfac74c264 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -2734,7 +2734,7 @@  static int coroutine_fn preallocate_co(BlockDriverState *bs, uint64_t offset,
     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) {