[Qemu-devel] [PATCH V4 4/4] Change default to qcow2 for sync mode none.

[Ian Main <imain@redhat.com>]

qcow2 supports backing files so it makes sense to default to qcow2
for MIRROR_SYNC_MODE_NONE so that we can use the source as a backing
drive and export it via nbd.  Defaulting FULL and TOP to SYNC_MODE_NONE
breaks tests but that could be fixed if we wanted it.

Signed-off-by: Ian Main <imain@redhat.com>
---
 blockdev.c       | 5 ++++-
 qapi-schema.json | 1 +
 2 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/blockdev.c b/blockdev.c
index 000dea6..a56ba08 100644
--- a/blockdev.c
+++ b/blockdev.c
@@ -1462,7 +1462,10 @@ void qmp_drive_backup(const char *device, const char *target,
     }
 
     if (!has_format) {
-        format = mode == NEW_IMAGE_MODE_EXISTING ? NULL : bs->drv->format_name;
+        format = NULL;
+        if (mode != NEW_IMAGE_MODE_EXISTING) {
+            format = sync == MIRROR_SYNC_MODE_NONE ? "qcow2" : bs->drv->format_name;
+        }
     }
     if (format) {
         drv = bdrv_find_format(format);
diff --git a/qapi-schema.json b/qapi-schema.json
index b3f6c2a..e2c86f9 100644
--- a/qapi-schema.json
+++ b/qapi-schema.json
@@ -1806,6 +1806,7 @@
 #
 # @format: #optional the format of the new destination, default is to
 #          probe if @mode is 'existing', else the format of the source
+#          drive.  If @sync is 'none' then the default is qcow2.
 #
 # @sync: what parts of the disk image should be copied to the destination
 #        (all the disk, only the sectors allocated in the topmost image, or
-- 
1.8.1.4