diff --git a/src/low_level.ml b/src/low_level.ml
index 1703cf9..6e10f09 100644
--- a/src/low_level.ml
+++ b/src/low_level.ml
@@ -560,10 +560,15 @@ let writev fd bufs =
         let size = e.stat.size in
         (* If the position is equal to the size then we just append. *)
         if Optint.Int63.equal size e.pos then
-          let contents = Cstruct.concat (e.contents :: bufs) in
-          let new_size = Optint.Int63.of_int @@ Cstruct.length contents in
-          let stat = { e.stat with size = new_size } in
-          r := File { e with stat; contents; pos = new_size }
+          (* We do it one buf at a time so we can simulate some concurrency... *)
+          List.iter
+            (fun buf ->
+              let contents = Cstruct.concat (e.contents :: [ buf ]) in
+              let new_size = Optint.Int63.of_int @@ Cstruct.length contents in
+              let stat = { e.stat with size = new_size } in
+              r := File { e with stat; contents; pos = new_size };
+              Eio.Fiber.yield ())
+            bufs
         else
           (* Position is less than the size, so need to merge contents. *)
           let src = Cstruct.concat bufs in
@@ -596,7 +601,8 @@ let readv fd bufs =
             let actual_len = if posi + len <= size then len else size - posi in
             Cstruct.blit e.contents (Optint.Int63.to_int !pos) buf 0 actual_len;
             pos := Optint.Int63.add !pos (Optint.Int63.of_int len);
-            read := !read + actual_len)
+            read := !read + actual_len;
+            Eio.Fiber.yield ())
           bufs;
         r := File { e with pos = !pos }
     | r -> Fmt.failwith "TODO: readv %a" Resource.pp r