block: Make backing files always writeback

First of all, we're generally not writing to backing files, but when we
do, it's in the context of block jobs which know very well when to flush
the image.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>

1 files changed, 3 insertions, 2 deletions

diff --git a/block.c b/block.c
index c4dca31fb2..d050c4bb58 100644
--- a/block.c
+++ b/block.c
@@ -738,8 +738,9 @@ static void bdrv_backing_options(int *child_flags, QDict *child_options,
 {
     int flags = parent_flags;
 
-    /* The cache mode is inherited unmodified for backing files */
-    qdict_copy_default(child_options, parent_options, BDRV_OPT_CACHE_WB);
+    /* The cache mode is inherited unmodified for backing files; except WCE,
+     * which is only applied on the top level (BlockBackend) */
+    qdict_set_default_str(child_options, BDRV_OPT_CACHE_WB, "on");
     qdict_copy_default(child_options, parent_options, BDRV_OPT_CACHE_DIRECT);
     qdict_copy_default(child_options, parent_options, BDRV_OPT_CACHE_NO_FLUSH);