The backing device is only modified from bdrv_commit. So instead of flushing it every time bdrv_flush is called for the front-end device only flush it after we're written data to it in bdrv_commit.
Signed-off-by: Christoph Hellwig <h...@lst.de> Index: qemu/block.c =================================================================== --- qemu.orig/block.c 2010-01-12 11:34:35.549024986 +0100 +++ qemu/block.c 2010-01-12 11:43:28.965006129 +0100 @@ -623,6 +623,12 @@ int bdrv_commit(BlockDriverState *bs) if (drv->bdrv_make_empty) return drv->bdrv_make_empty(bs); + /* + * Make sure all data we wrote to the backing device is actually + * stable on disk. + */ + if (bs->backing_hd) + bdrv_flush(bs->backing_hd); return 0; } @@ -1124,12 +1130,8 @@ const char *bdrv_get_device_name(BlockDr void bdrv_flush(BlockDriverState *bs) { - if (!bs->drv) - return; - if (bs->drv->bdrv_flush) + if (bs->drv && bs->drv->bdrv_flush) bs->drv->bdrv_flush(bs); - if (bs->backing_hd) - bdrv_flush(bs->backing_hd); } void bdrv_flush_all(void) @@ -1806,11 +1808,6 @@ BlockDriverAIOCB *bdrv_aio_flush(BlockDr if (!drv) return NULL; - - /* - * Note that unlike bdrv_flush the driver is reponsible for flushing a - * backing image if it exists. - */ return drv->bdrv_aio_flush(bs, cb, opaque); }