diff --git a/block.c b/block.c
index 985d0b7..6b505fb 100644
--- a/block.c
+++ b/block.c
@@ -1453,14 +1453,27 @@ const char *bdrv_get_device_name(BlockDriverState *bs)
     return bs->device_name;
 }
 
-void bdrv_flush(BlockDriverState *bs)
+int bdrv_flush(BlockDriverState *bs)
 {
     if (bs->open_flags & BDRV_O_NO_FLUSH) {
-        return;
+        return 0;
+    }
+
+    if (bs->drv && bs->drv->bdrv_flush) {
+        return bs->drv->bdrv_flush(bs);
     }
 
-    if (bs->drv && bs->drv->bdrv_flush)
-        bs->drv->bdrv_flush(bs);
+    /*
+     * Some block drivers always operate in either writethrough or unsafe mode
+     * and don't support bdrv_flush therefore. Usually qemu doesn't know how
+     * the server works (because the behaviour is hardcoded or depends on
+     * server-side configuration), so we can't ensure that everything is safe
+     * on disk. Returning an error doesn't work because that would break guests
+     * even if the server operates in writethrough mode.
+     *
+     * Let's hope the user knows what he's doing.
+     */
+    return 0;
 }
 
 void bdrv_flush_all(void)
diff --git a/block.h b/block.h
index a4facf2..78ecfac 100644
--- a/block.h
+++ b/block.h
@@ -142,7 +142,7 @@ BlockDriverAIOCB *bdrv_aio_ioctl(BlockDriverState *bs,
         BlockDriverCompletionFunc *cb, void *opaque);
 
 /* Ensure contents are flushed to disk.  */
-void bdrv_flush(BlockDriverState *bs);
+int bdrv_flush(BlockDriverState *bs);
 void bdrv_flush_all(void);
 void bdrv_close_all(void);
 
diff --git a/block/blkdebug.c b/block/blkdebug.c
index 4d6ff0a..cd9eb80 100644
--- a/block/blkdebug.c
+++ b/block/blkdebug.c
@@ -397,9 +397,9 @@ static void blkdebug_close(BlockDriverState *bs)
     }
 }
 
-static void blkdebug_flush(BlockDriverState *bs)
+static int blkdebug_flush(BlockDriverState *bs)
 {
-    bdrv_flush(bs->file);
+    return bdrv_flush(bs->file);
 }
 
 static BlockDriverAIOCB *blkdebug_aio_flush(BlockDriverState *bs,
diff --git a/block/blkverify.c b/block/blkverify.c
index b2a12fe..0a8d691 100644
--- a/block/blkverify.c
+++ b/block/blkverify.c
@@ -116,12 +116,12 @@ static void blkverify_close(BlockDriverState *bs)
     s->test_file = NULL;
 }
 
-static void blkverify_flush(BlockDriverState *bs)
+static int blkverify_flush(BlockDriverState *bs)
 {
     BDRVBlkverifyState *s = bs->opaque;
 
     /* Only flush test file, the raw file is not important */
-    bdrv_flush(s->test_file);
+    return bdrv_flush(s->test_file);
 }
 
 static int64_t blkverify_getlength(BlockDriverState *bs)
diff --git a/block/cow.c b/block/cow.c
index eedcc48..4cf543c 100644
--- a/block/cow.c
+++ b/block/cow.c
@@ -282,9 +282,9 @@ exit:
     return ret;
 }
 
-static void cow_flush(BlockDriverState *bs)
+static int cow_flush(BlockDriverState *bs)
 {
-    bdrv_flush(bs->file);
+    return bdrv_flush(bs->file);
 }
 
 static QEMUOptionParameter cow_create_options[] = {
diff --git a/block/qcow.c b/block/qcow.c
index 816103d..9cd547d 100644
--- a/block/qcow.c
+++ b/block/qcow.c
@@ -910,9 +910,9 @@ static int qcow_write_compressed(BlockDriverState *bs, int64_t sector_num,
     return 0;
 }
 
-static void qcow_flush(BlockDriverState *bs)
+static int qcow_flush(BlockDriverState *bs)
 {
-    bdrv_flush(bs->file);
+    return bdrv_flush(bs->file);
 }
 
 static BlockDriverAIOCB *qcow_aio_flush(BlockDriverState *bs,
diff --git a/block/qcow2.c b/block/qcow2.c
index b816d87..537c479 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -1148,9 +1148,9 @@ static int qcow_write_compressed(BlockDriverState *bs, int64_t sector_num,
     return 0;
 }
 
-static void qcow_flush(BlockDriverState *bs)
+static int qcow_flush(BlockDriverState *bs)
 {
-    bdrv_flush(bs->file);
+    return bdrv_flush(bs->file);
 }
 
 static BlockDriverAIOCB *qcow_aio_flush(BlockDriverState *bs,
diff --git a/block/raw-posix.c b/block/raw-posix.c
index d0393e0..d0960b8 100644
--- a/block/raw-posix.c
+++ b/block/raw-posix.c
@@ -734,10 +734,10 @@ static int raw_create(const char *filename, QEMUOptionParameter *options)
     return result;
 }
 
-static void raw_flush(BlockDriverState *bs)
+static int raw_flush(BlockDriverState *bs)
 {
     BDRVRawState *s = bs->opaque;
-    qemu_fdatasync(s->fd);
+    return qemu_fdatasync(s->fd);
 }
 
 
diff --git a/block/raw-win32.c b/block/raw-win32.c
index 503ed39..7f32778 100644
--- a/block/raw-win32.c
+++ b/block/raw-win32.c
@@ -150,7 +150,14 @@ static int raw_write(BlockDriverState *bs, int64_t sector_num,
 static void raw_flush(BlockDriverState *bs)
 {
     BDRVRawState *s = bs->opaque;
-    FlushFileBuffers(s->hfile);
+    int ret;
+
+    ret = FlushFileBuffers(s->hfile);
+    if (ret != 0) {
+        return -EIO;
+    }
+
+    return 0;
 }
 
 static void raw_close(BlockDriverState *bs)
diff --git a/block/raw.c b/block/raw.c
index 9108779..1980deb 100644
--- a/block/raw.c
+++ b/block/raw.c
@@ -39,9 +39,9 @@ static void raw_close(BlockDriverState *bs)
 {
 }
 
-static void raw_flush(BlockDriverState *bs)
+static int raw_flush(BlockDriverState *bs)
 {
-    bdrv_flush(bs->file);
+    return bdrv_flush(bs->file);
 }
 
 static BlockDriverAIOCB *raw_aio_flush(BlockDriverState *bs,
diff --git a/block/vdi.c b/block/vdi.c
index f72633c..3b51e53 100644
--- a/block/vdi.c
+++ b/block/vdi.c
@@ -900,10 +900,10 @@ static void vdi_close(BlockDriverState *bs)
 {
 }
 
-static void vdi_flush(BlockDriverState *bs)
+static int vdi_flush(BlockDriverState *bs)
 {
     logout("\n");
-    bdrv_flush(bs->file);
+    return bdrv_flush(bs->file);
 }
 
 
diff --git a/block/vmdk.c b/block/vmdk.c
index 2d4ba42..872aeba 100644
--- a/block/vmdk.c
+++ b/block/vmdk.c
@@ -823,9 +823,9 @@ static void vmdk_close(BlockDriverState *bs)
     qemu_free(s->l2_cache);
 }
 
-static void vmdk_flush(BlockDriverState *bs)
+static int vmdk_flush(BlockDriverState *bs)
 {
-    bdrv_flush(bs->file);
+    return bdrv_flush(bs->file);
 }
 
 
diff --git a/block_int.h b/block_int.h
index 87e60b8..3c3adb5 100644
--- a/block_int.h
+++ b/block_int.h
@@ -59,7 +59,7 @@ struct BlockDriver {
                       const uint8_t *buf, int nb_sectors);
     void (*bdrv_close)(BlockDriverState *bs);
     int (*bdrv_create)(const char *filename, QEMUOptionParameter *options);
-    void (*bdrv_flush)(BlockDriverState *bs);
+    int (*bdrv_flush)(BlockDriverState *bs);
     int (*bdrv_is_allocated)(BlockDriverState *bs, int64_t sector_num,
                              int nb_sectors, int *pnum);
     int (*bdrv_set_key)(BlockDriverState *bs, const char *key);
