[Qemu-devel] [PATCH] qcow2: Fix metadata preallocation

[Qemu-devel] [PATCH] qcow2: Fix metadata preallocation

[Kevin Wolf]

The wrong version of the preallocation patch has been applied, so this is the
remaining diff.

We can't use truncate to grow the image file to the right size because we don't
know if metadata has been written after the last data cluster. In this case
truncate would shrink the file and destroy its metadata. Write a zero sector at
the end of the virtual disk instead to ensure that the file is big enough.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
---
 block/qcow2.c |    4 +++-
 1 files changed, 3 insertions(+), 1 deletions(-)

diff --git a/block/qcow2.c b/block/qcow2.c
index 9637f2e..b8eae90 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -677,7 +677,9 @@ static int preallocate(BlockDriverState *bs)
      * EOF). Extend the image to the last allocated sector.
      */
     if (cluster_offset != 0) {
-        bdrv_truncate(s->hd, cluster_offset + (num <<  9));
+        uint8_t buf[512];
+        memset(buf, 0, 512);
+        bdrv_write(s->hd, (cluster_offset >> 9) + num - 1, buf, 1);
     }
 
     return 0;
-- 
1.6.0.6