nbd: use TCP_CORK in nbd_co_send_request()

Use TCP_CORK to defer packet transmission until both the header and the
payload have been written.

Suggested-by: Nick Thomas <nick@bytemark.co.uk>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 16 insertions, 6 deletions

diff --git a/block/nbd.c b/block/nbd.c
index 662df16..485bbf0 100644
--- a/block/nbd.c
+++ b/block/nbd.c
@@ -334,13 +334,23 @@ static int nbd_co_send_request(BDRVNBDState *s, struct nbd_request *request,
     s->send_coroutine = qemu_coroutine_self();
     qemu_aio_set_fd_handler(s->sock, nbd_reply_ready, nbd_restart_write,
                             nbd_have_request, s);
-    rc = nbd_send_request(s->sock, request);
-    if (rc >= 0 && qiov) {
-        ret = qemu_co_sendv(s->sock, qiov->iov, qiov->niov,
-                            offset, request->len);
-        if (ret != request->len) {
-            rc = -EIO;
+    if (qiov) {
+        if (!s->is_unix) {
+            socket_set_cork(s->sock, 1);
         }
+        rc = nbd_send_request(s->sock, request);
+        if (rc >= 0) {
+            ret = qemu_co_sendv(s->sock, qiov->iov, qiov->niov,
+                                offset, request->len);
+            if (ret != request->len) {
+                rc = -EIO;
+            }
+        }
+        if (!s->is_unix) {
+            socket_set_cork(s->sock, 0);
+        }
+    } else {
+        rc = nbd_send_request(s->sock, request);
     }
     qemu_aio_set_fd_handler(s->sock, nbd_reply_ready, NULL,
                             nbd_have_request, s);