diff --git a/src/client.c b/src/client.c
index 5653a9b..2597ca6 100644
--- a/src/client.c
+++ b/src/client.c
@@ -132,6 +132,12 @@ void write_not_zeroes(struct client* client, uint64_t from, uint64_t len)
 			debug("writing the lot: from=%ld, run=%d", from, run);
 			/* already allocated, just write it all */
 			DO_READ(client->mapped + from, run);
+			/* We know from our earlier call to  bitset_run_count that the
+			 * bitset is all-1s at this point, but we need to dirty it for the
+			 * sake of the event stream - the actual bytes have changed, and we
+			 * are interested in that fact.
+			 */
+			bitset_set_range( map, from, run );
 			server_dirty(client->serve, from, run);
 			len  -= run;
 			from += run;
@@ -167,6 +173,10 @@ void write_not_zeroes(struct client* client, uint64_t from, uint64_t len)
 					 * sparseness as possible.
 					 */
 				}
+				/* When the block is all_zeroes, no bytes have changed, so we
+				 * don't need to put an event into the bitset stream. This may
+				 * be surprising in the future.
+				 */
 
 				len  -= blockrun;
 				run  -= blockrun;
diff --git a/src/serve.c b/src/serve.c
index a6e29b1..e03812d 100644
--- a/src/serve.c
+++ b/src/serve.c
@@ -705,6 +705,8 @@ void* build_allocation_map_thread(void* serve_uncast)
 		 * All that happens if we leave it is that it gradually builds up an
 		 * *incomplete* record of writes. Nobody will use it, as
 		 * allocation_map_built == 0 for the lifetime of the process.
+		 *
+		 * The stream functionality can still be relied on.
 		 */
 		warn( "Didn't build allocation map for %s", serve->filename );
 	}
diff --git a/tests/acceptance/test_serve_mode.rb b/tests/acceptance/test_serve_mode.rb
index 1278e68..b0eca04 100644
--- a/tests/acceptance/test_serve_mode.rb
+++ b/tests/acceptance/test_serve_mode.rb
@@ -44,6 +44,29 @@ class TestServeMode < Test::Unit::TestCase
     end
   end
 
+  def test_long_write_on_top_of_short_write_is_respected
+
+    connect_to_server do |client|
+      # Start with a file of all-zeroes.
+      client.write( 0, "\x00" * @env.file1.size )
+      rsp = client.read_response
+      assert_equal FlexNBD::REPLY_MAGIC, rsp[:magic]
+      assert_equal 0, rsp[:error]
+
+      client.write( 0, "\xFF" )
+      rsp = client.read_response
+      assert_equal FlexNBD::REPLY_MAGIC, rsp[:magic]
+      assert_equal 0, rsp[:error]
+
+      client.write( 0, "\xFF\xFF" )
+      rsp = client.read_response
+      assert_equal FlexNBD::REPLY_MAGIC, rsp[:magic]
+      assert_equal 0, rsp[:error]
+    end
+
+    assert_equal "\xFF\xFF", @env.file1.read (0, 2 )
+  end
+
 
   def test_read_request_out_of_bounds_receives_error_response
     connect_to_server do |client|