[Qemu-devel] [PATCH v2] qcow2: Flush image after creation

[Max Reitz <mreitz@redhat.com>]

Opening the qcow2 image with BDRV_O_NO_FLUSH prevents any flushes during
the image creation. This means that the image has not yet been flushed
to disk when qemu-img create exits. This flush is delayed until the next
operation on the image involving opening it without BDRV_O_NO_FLUSH and
closing (or directly flushing) it. For large images and/or images with a
small cluster size and preallocated metadata, this flush may take a
significant amount of time and may occur unexpectedly.

Reopening the image without BDRV_O_NO_FLUSH right before the end of
qcow2_create2() results in hoisting the potentially costly flush into
the image creation, which is expected to take some time (whereas
successive image operations may be not).

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
---
v2:
 - implemented Kevin's remarks:
   - left ret = 0; in the code
   - added goto out; in error handling of bdrv_open
 - implemented Eric's remarks:
   - changed indentation of bdrv_open arguments
   - s/preponing/hoisting/ in the commit message
---
 block/qcow2.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/block/qcow2.c b/block/qcow2.c
index 01269f9..6e5d98d 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -1584,6 +1584,16 @@ static int qcow2_create2(const char *filename, int64_t total_size,
         }
     }
 
+    bdrv_close(bs);
+
+    /* Reopen the image without BDRV_O_NO_FLUSH to flush it before returning */
+    ret = bdrv_open(bs, filename, NULL,
+                    BDRV_O_RDWR | BDRV_O_CACHE_WB, drv, &local_err);
+    if (error_is_set(&local_err)) {
+        error_propagate(errp, local_err);
+        goto out;
+    }
+
     ret = 0;
 out:
     bdrv_unref(bs);
-- 
1.8.3.1