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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>
This commit is contained in:
Kevin Wolf 2019-04-15 16:25:01 +02:00
parent de38b5005e
commit f29fbf7c6b
1 changed files with 1 additions and 1 deletions
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2
block/qcow2.c
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@ -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) {