[Qemu-devel] [PATCH 27/36] qcow2: Avoid COW during metadata preallocation

[Michael Roth]

From: Kevin Wolf <address@hidden>

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: address@hidden
Signed-off-by: Kevin Wolf <address@hidden>
Reviewed-by: Eric Blake <address@hidden>
(cherry picked from commit f29fbf7c6b1c9a84f6931c1c222716fbe073e6e4)
*modified to avoid functional dependency on 93e32b3e
Signed-off-by: Michael Roth <address@hidden>
---
 block/qcow2.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/block/qcow2.c b/block/qcow2.c
index 991d6ac91b..6ddaf9a1f8 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -2590,6 +2590,7 @@ static int qcow2_set_up_encryption(BlockDriverState *bs,
 static int coroutine_fn preallocate_co(BlockDriverState *bs, uint64_t offset,
                                        uint64_t new_length)
 {
+    BDRVQcow2State *s = bs->opaque;
     uint64_t bytes;
     uint64_t host_offset = 0;
     unsigned int cur_bytes;
@@ -2600,7 +2601,7 @@ 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) {
-- 
2.17.1