diff --git a/services/sync/modules/engines.js b/services/sync/modules/engines.js
index f0ca10f7e332..f63f43586652 100644
--- a/services/sync/modules/engines.js
+++ b/services/sync/modules/engines.js
@@ -77,9 +77,19 @@ Tracker.prototype = {
     this._score = 0;
   },
 
-  saveChangedIDs: function T_saveChangedIDs() {
+  persistChangedIDs: true,
+
+  /**
+   * Persist changedIDs to disk at a later date.
+   * Optionally pass a callback to be invoked when the write has occurred.
+   */
+  saveChangedIDs: function (cb) {
+    if (!this.persistChangedIDs) {
+      this._log.debug("Not saving changedIDs.");
+      return;
+    }
     Utils.namedTimer(function() {
-      Utils.jsonSave("changes/" + this.file, this, this.changedIDs);
+      Utils.jsonSave("changes/" + this.file, this, this.changedIDs, cb);
     }, 1000, this, "_lazySave");
   },
 
@@ -122,7 +132,7 @@ Tracker.prototype = {
     if ((this.changedIDs[id] || -Infinity) < when) {
       this._log.trace("Adding changed ID: " + [id, when]);
       this.changedIDs[id] = when;
-      this.saveChangedIDs();
+      this.saveChangedIDs(this.onSavedChangedIDs);
     }
     return true;
   },
diff --git a/services/sync/tests/unit/test_addons_tracker.js b/services/sync/tests/unit/test_addons_tracker.js
index b0a4a894bbbf..65950e3553eb 100644
--- a/services/sync/tests/unit/test_addons_tracker.js
+++ b/services/sync/tests/unit/test_addons_tracker.js
@@ -20,6 +20,9 @@ let reconciler = engine._reconciler;
 let store      = engine._store;
 let tracker    = engine._tracker;
 
+// Don't write out by default.
+tracker.persistChangedIDs = false;
+
 const addon1ID = "addon1@tests.mozilla.org";
 
 function cleanup_and_advance() {
@@ -28,7 +31,6 @@ function cleanup_and_advance() {
 
   tracker.resetScore();
   tracker.clearChangedIDs();
-  tracker._lazySave.clear();
 
   reconciler._addons = {};
   reconciler._changes = [];
diff --git a/services/sync/tests/unit/test_bookmark_store.js b/services/sync/tests/unit/test_bookmark_store.js
index dac10bc6960d..578eb142a0d0 100644
--- a/services/sync/tests/unit/test_bookmark_store.js
+++ b/services/sync/tests/unit/test_bookmark_store.js
@@ -9,8 +9,14 @@ Cu.import("resource://services-sync/util.js");
 const PARENT_ANNO = "sync/parent";
 
 Service.engineManager.register(BookmarksEngine);
+
 let engine = Service.engineManager.get("bookmarks");
 let store = engine._store;
+let tracker = engine._tracker;
+
+// Don't write some persistence files asynchronously.
+tracker.persistChangedIDs = false;
+
 let fxuri = Utils.makeURI("http://getfirefox.com/");
 let tburi = Utils.makeURI("http://getthunderbird.com/");
 
@@ -324,9 +330,9 @@ add_test(function test_move_order() {
     toolbar.children = [bmk2_guid, bmk1_guid];
     store.applyIncoming(toolbar);
     // Bookmarks engine does this at the end of _processIncoming
-    engine._tracker.ignoreAll = true;
+    tracker.ignoreAll = true;
     store._orderChildren();
-    engine._tracker.ignoreAll = false;
+    tracker.ignoreAll = false;
     delete store._childrenToOrder;
 
     _("Verify new order.");
@@ -399,11 +405,6 @@ add_test(function test_reparentOrphans() {
   } finally {
     _("Clean up.");
     store.wipe();
-
-    if (engine._tracker._lazySave) {
-      engine._tracker._lazySave.clear();
-    }
-
     run_next_test();
   }
 });
diff --git a/services/sync/tests/unit/test_bookmark_tracker.js b/services/sync/tests/unit/test_bookmark_tracker.js
index c517f89335cb..1ab43a47f322 100644
--- a/services/sync/tests/unit/test_bookmark_tracker.js
+++ b/services/sync/tests/unit/test_bookmark_tracker.js
@@ -11,7 +11,10 @@ Cu.import("resource://services-sync/util.js");
 Service.engineManager.register(BookmarksEngine);
 let engine = Service.engineManager.get("bookmarks");
 let store  = engine._store;
+let tracker = engine._tracker;
+
 store.wipe();
+tracker.persistChangedIDs = false;
 
 function test_tracking() {
   _("Verify we've got an empty tracker to work with.");
@@ -66,7 +69,6 @@ function test_tracking() {
     store.wipe();
     tracker.clearChangedIDs();
     tracker.resetScore();
-    tracker._lazySave.clear();
     Svc.Obs.notify("weave:engine:stop-tracking");
   }
 }
@@ -105,7 +107,6 @@ function test_onItemChanged() {
     store.wipe();
     tracker.clearChangedIDs();
     tracker.resetScore();
-    tracker._lazySave.clear();
     Svc.Obs.notify("weave:engine:stop-tracking");
   }
 }
@@ -158,7 +159,6 @@ function test_onItemMoved() {
     store.wipe();
     tracker.clearChangedIDs();
     tracker.resetScore();
-    tracker._lazySave.clear();
     Svc.Obs.notify("weave:engine:stop-tracking");
   }
 
diff --git a/services/sync/tests/unit/test_forms_tracker.js b/services/sync/tests/unit/test_forms_tracker.js
index dfb1081c269b..bc943e013856 100644
--- a/services/sync/tests/unit/test_forms_tracker.js
+++ b/services/sync/tests/unit/test_forms_tracker.js
@@ -9,6 +9,9 @@ Cu.import("resource://services-sync/util.js");
 function run_test() {
   _("Verify we've got an empty tracker to work with.");
   let tracker = new FormEngine(Service)._tracker;
+  // Don't do asynchronous writes.
+  tracker.persistChangedIDs = false;
+
   do_check_empty(tracker.changedIDs);
   Log4Moz.repository.rootLogger.addAppender(new Log4Moz.DumpAppender());
 
@@ -46,8 +49,5 @@ function run_test() {
   } finally {
     _("Clean up.");
     Svc.Form.removeAllEntries();
-    if (tracker._lazySave) {
-      tracker._lazySave.clear();
-    }
   }
 }
diff --git a/services/sync/tests/unit/test_history_tracker.js b/services/sync/tests/unit/test_history_tracker.js
index 8c493ea3e4db..d6f6fc9baa1a 100644
--- a/services/sync/tests/unit/test_history_tracker.js
+++ b/services/sync/tests/unit/test_history_tracker.js
@@ -24,6 +24,9 @@ Service.engineManager.register(HistoryEngine);
 let engine = Service.engineManager.get("history");
 let tracker = engine._tracker;
 
+// Don't write out by default.
+tracker.persistChangedIDs = false;
+
 let _counter = 0;
 function addVisit() {
   let uri = Utils.makeURI("http://getfirefox.com/" + _counter);
@@ -55,12 +58,23 @@ add_test(function test_not_tracking(next) {
 });
 
 add_test(function test_start_tracking() {
+  _("Add hook for save completion.");
+  tracker.persistChangedIDs = true;
+  tracker.onSavedChangedIDs = function () {
+    _("changedIDs written to disk. Proceeding.");
+    // Turn this back off.
+    tracker.persistChangedIDs = false;
+    delete tracker.onSavedChangedIDs;
+    run_next_test();
+  };
+
   _("Tell the tracker to start tracking changes.");
   onScoreUpdated(function() {
     do_check_attribute_count(tracker.changedIDs, 1);
     do_check_eq(tracker.score, SCORE_INCREMENT_SMALL);
     run_next_test();
   });
+
   Svc.Obs.notify("weave:engine:start-tracking");
   addVisit();
 });
@@ -148,8 +162,5 @@ add_test(function test_stop_tracking_twice() {
 add_test(function cleanup() {
    _("Clean up.");
   PlacesUtils.history.removeAllPages();
-  if (tracker._lazySave) {
-    tracker._lazySave.clear();
-  }
   run_next_test();
 });
diff --git a/services/sync/tests/unit/test_password_tracker.js b/services/sync/tests/unit/test_password_tracker.js
index 567621ea0d6d..ddfc524ab044 100644
--- a/services/sync/tests/unit/test_password_tracker.js
+++ b/services/sync/tests/unit/test_password_tracker.js
@@ -10,12 +10,20 @@ Cu.import("resource://services-sync/util.js");
 Service.engineManager.register(PasswordEngine);
 let engine = Service.engineManager.get("passwords");
 let store  = engine._store;
+let tracker = engine._tracker;
 
-function test_tracking() {
+// Don't do asynchronous writes.
+tracker.persistChangedIDs = false;
+
+function run_test() {
+  initTestLogging("Trace");
+  run_next_test();
+}
+
+add_test(function test_tracking() {
   let recordNum = 0;
 
   _("Verify we've got an empty tracker to work with.");
-  let tracker = engine._tracker;
   do_check_empty(tracker.changedIDs);
 
   function createPassword() {
@@ -69,14 +77,13 @@ function test_tracking() {
     store.wipe();
     tracker.clearChangedIDs();
     tracker.resetScore();
-    tracker._lazySave.clear();
     Svc.Obs.notify("weave:engine:stop-tracking");
+    run_next_test();
   }
-}
+});
 
-function test_onWipe() {
+add_test(function test_onWipe() {
   _("Verify we've got an empty tracker to work with.");
-  let tracker = engine._tracker;
   do_check_empty(tracker.changedIDs);
   do_check_eq(tracker.score, 0);
 
@@ -89,16 +96,6 @@ function test_onWipe() {
   } finally {
     tracker.resetScore();
     Svc.Obs.notify("weave:engine:stop-tracking");
+    run_next_test();
   }
-}
-
-function run_test() {
-  initTestLogging("Trace");
-
-  Log4Moz.repository.getLogger("Sync.Engine.Passwords").level = Log4Moz.Level.Trace;
-  Log4Moz.repository.getLogger("Sync.Store.Passwords").level = Log4Moz.Level.Trace;
-  Log4Moz.repository.getLogger("Sync.Tracker.Passwords").level = Log4Moz.Level.Trace;
-
-  test_tracking();
-  test_onWipe();
-}
+});
diff --git a/services/sync/tests/unit/test_prefs_tracker.js b/services/sync/tests/unit/test_prefs_tracker.js
index 9f91ca0fa76f..dd45f239852f 100644
--- a/services/sync/tests/unit/test_prefs_tracker.js
+++ b/services/sync/tests/unit/test_prefs_tracker.js
@@ -11,6 +11,10 @@ Cu.import("resource://services-sync/util.js");
 function run_test() {
   let engine = Service.engineManager.get("prefs");
   let tracker = engine._tracker;
+
+  // Don't write out by default.
+  tracker.persistChangedIDs = false;
+
   let prefs = new Preferences();
 
   try {
@@ -80,8 +84,5 @@ function run_test() {
   } finally {
     Svc.Obs.notify("weave:engine:stop-tracking");
     prefs.resetBranch("");
-    if (tracker._lazySave) {
-      tracker._lazySave.clear();
-    }
   }
 }
diff --git a/services/sync/tests/unit/test_tab_tracker.js b/services/sync/tests/unit/test_tab_tracker.js
index 3d9d54a62ab1..230dc68c14bc 100644
--- a/services/sync/tests/unit/test_tab_tracker.js
+++ b/services/sync/tests/unit/test_tab_tracker.js
@@ -51,6 +51,8 @@ function run_test() {
 
   _("We assume that tabs have changed at startup.");
   let tracker = engine._tracker;
+  tracker.persistChangedIDs = false;
+
   do_check_true(tracker.modified);
   do_check_true(Utils.deepEquals(Object.keys(engine.getChangedIDs()),
                                  [clientsEngine.localID]));
@@ -113,7 +115,4 @@ function run_test() {
   do_check_true(Utils.deepEquals(Object.keys(engine.getChangedIDs()),
                                  [clientsEngine.localID]));
   do_check_eq(logs.length, idx); // test that setTabValue isn't called
-  if (tracker._lazySave) {
-    tracker._lazySave.clear();
-  }
 }
diff --git a/services/sync/tests/unit/test_tracker_addChanged.js b/services/sync/tests/unit/test_tracker_addChanged.js
index 7f8e3fa039eb..e73bd1162f54 100644
--- a/services/sync/tests/unit/test_tracker_addChanged.js
+++ b/services/sync/tests/unit/test_tracker_addChanged.js
@@ -3,9 +3,16 @@
 
 Cu.import("resource://services-sync/engines.js");
 Cu.import("resource://services-sync/service.js");
+Cu.import("resource://services-sync/util.js");
 
 function run_test() {
+  run_next_test();
+}
+
+add_test(function test_tracker_basics() {
   let tracker = new Tracker("Tracker", Service);
+  tracker.persistChangedIDs = false;
+
   let id = "the_id!";
 
   _("Make sure nothing exists yet..");
@@ -26,5 +33,27 @@ function run_test() {
   _("Adding without time defaults to current time");
   tracker.addChangedID(id);
   do_check_true(tracker.changedIDs[id] > 10);
-  tracker._lazySave.clear()
-}
+
+  run_next_test();
+});
+
+add_test(function test_tracker_persistence() {
+  let tracker = new Tracker("Tracker", Service);
+  let id = "abcdef";
+
+  tracker.persistChangedIDs = true;
+  tracker.onSavedChangedIDs = function () {
+    _("IDs saved.");
+    do_check_eq(5, tracker.changedIDs[id]);
+
+    // Verify the write by reading the file back.
+    Utils.jsonLoad("changes/tracker", this, function (json) {
+      do_check_eq(5, json[id]);
+      tracker.persistChangedIDs = false;
+      delete tracker.onSavedChangedIDs;
+      run_next_test();
+    });
+  };
+
+  tracker.addChangedID(id, 5);
+});