[Qemu-devel] [PATCH v3 27/44] nbd: Use BDRV_REQ_FUA for better FUA where supported

[Eric Blake <eblake@redhat.com>]

Rather than always flushing ourselves, let the block layer
forward the FUA on to the underlying device - where all
layers understand FUA, we are now more efficient; and where
the underlying layer doesn't understand it, now the block
layer takes care of the full flush fallback on our behalf.

Signed-off-by: Eric Blake <eblake@redhat.com>
---
 nbd/server.c | 16 ++++++----------
 1 file changed, 6 insertions(+), 10 deletions(-)

diff --git a/nbd/server.c b/nbd/server.c
index 9be0a99..fa05a73 100644
--- a/nbd/server.c
+++ b/nbd/server.c
@@ -1085,6 +1085,7 @@ static void nbd_trip(void *opaque)
     struct nbd_reply reply;
     ssize_t ret;
     uint32_t command;
+    int flags;

     TRACE("Reading request.");
     if (client->closing) {
@@ -1153,23 +1154,18 @@ static void nbd_trip(void *opaque)

         TRACE("Writing to device");

+        flags = 0;
+        if (request.type & NBD_CMD_FLAG_FUA) {
+            flags |= BDRV_REQ_FUA;
+        }
         ret = blk_pwrite(exp->blk, request.from + exp->dev_offset,
-                         req->data, request.len, 0);
+                         req->data, request.len, flags);
         if (ret < 0) {
             LOG("writing to file failed");
             reply.error = -ret;
             goto error_reply;
         }

-        if (request.type & NBD_CMD_FLAG_FUA) {
-            ret = blk_co_flush(exp->blk);
-            if (ret < 0) {
-                LOG("flush failed");
-                reply.error = -ret;
-                goto error_reply;
-            }
-        }
-
         if (nbd_co_send_reply(req, &reply, 0) < 0) {
             goto out;
         }
-- 
2.5.5