guard deletes on disk tables in order to eliminate superfluos fsyncs

Unfortunately this makes queue deletion O(binding_count^2), so further
work is needed.

3 files changed, 23 insertions, 5 deletions

diff --git a/src/rabbit_amqqueue.erl b/src/rabbit_amqqueue.erl
index d566ac87..b2473f91 100644
--- a/src/rabbit_amqqueue.erl
+++ b/src/rabbit_amqqueue.erl
@@ -578,7 +578,12 @@ flush_all(QPids, ChPid) ->
 
 internal_delete1(QueueName) ->
     ok = mnesia:delete({rabbit_queue, QueueName}),
-    ok = mnesia:delete({rabbit_durable_queue, QueueName}),
+    %% this 'guarded' delete prevents unnecessary writes to the mnesia
+    %% disk log
+    case mnesia:wread({rabbit_durable_queue, QueueName}) of
+        []  -> ok;
+        [_] -> ok = mnesia:delete({rabbit_durable_queue, QueueName})
+    end,
     %% we want to execute some things, as decided by rabbit_exchange,
     %% after the transaction.
     rabbit_binding:remove_for_destination(QueueName).
diff --git a/src/rabbit_binding.erl b/src/rabbit_binding.erl
index f0ea514d..540e221f 100644
--- a/src/rabbit_binding.erl
+++ b/src/rabbit_binding.erl
@@ -281,17 +281,17 @@ remove_for_source(SrcName) ->
                mnesia:match_object(rabbit_route, Match, write) ++
                    mnesia:match_object(rabbit_durable_route, Match, write)),
     [begin
-         sync_route(Route, fun mnesia:delete_object/3),
+         sync_route(Route, fun delete_object/3),
          Route#route.binding
      end || Route <- Routes].
 
 remove_for_destination(Dst) ->
     remove_for_destination(
-      Dst, fun (R) -> sync_route(R, fun mnesia:delete_object/3) end).
+      Dst, fun (R) -> sync_route(R, fun delete_object/3) end).
 
 remove_transient_for_destination(Dst) ->
     remove_for_destination(
-      Dst, fun (R) -> sync_transient_route(R, fun mnesia:delete_object/3) end).
+      Dst, fun (R) -> sync_transient_route(R, fun delete_object/3) end).
 
 %%----------------------------------------------------------------------------
 
@@ -308,6 +308,14 @@ binding_action(Binding = #binding{source      = SrcName,
               Fun(Src, Dst, Binding#binding{args = SortedArgs})
       end).
 
+delete_object(Tab, Record, LockKind) ->
+    %% this 'guarded' delete prevents unnecessary writes to the mnesia
+    %% disk log
+    case mnesia:match_object(Tab, Record, LockKind) of
+        []  -> ok;
+        [_] -> mnesia:delete_object(Tab, Record, LockKind)
+    end.
+
 sync_route(R, Fun) -> sync_route(R, true, true, Fun).
 
 sync_route(Route, true, true, Fun) ->
diff --git a/src/rabbit_exchange.erl b/src/rabbit_exchange.erl
index 57c571f1..4cc96ef5 100644
--- a/src/rabbit_exchange.erl
+++ b/src/rabbit_exchange.erl
@@ -402,7 +402,12 @@ conditional_delete(X = #exchange{name = XName}) ->
     end.
 
 unconditional_delete(X = #exchange{name = XName}) ->
-    ok = mnesia:delete({rabbit_durable_exchange, XName}),
+    %% this 'guarded' delete prevents unnecessary writes to the mnesia
+    %% disk log
+    case mnesia:wread({rabbit_durable_exchange, XName}) of
+        []  -> ok;
+        [_] -> ok = mnesia:delete({rabbit_durable_exchange, XName})
+    end,
     ok = mnesia:delete({rabbit_exchange, XName}),
     ok = mnesia:delete({rabbit_exchange_serial, XName}),
     Bindings = rabbit_binding:remove_for_source(XName),