Patchwork [1/4] qcow2: Avoid COW during metadata preallocation

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Submitter Kevin Wolf
Date April 15, 2019, 3:54 p.m.
Message ID <20190415155452.5115-2-kwolf@redhat.com>
Download mbox | patch
Permalink /patch/773337/
State New
Headers show

Comments

Kevin Wolf - April 15, 2019, 3:54 p.m.
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>
---
 block/qcow2.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)
Eric Blake - April 15, 2019, 4:14 p.m.
On 4/15/19 10:54 AM, Kevin Wolf wrote:
> 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>
> ---
>  block/qcow2.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)

Reviewed-by: Eric Blake <eblake@redhat.com>

Patch

diff --git a/block/qcow2.c b/block/qcow2.c
index d507ee0686..c8400e9712 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -2723,6 +2723,7 @@  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;
@@ -2733,7 +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) {