Index: jackrabbit-core/src/main/java/org/apache/jackrabbit/core/data/GarbageCollector.java
===================================================================
--- jackrabbit-core/src/main/java/org/apache/jackrabbit/core/data/GarbageCollector.java	(revision 1461410)
+++ jackrabbit-core/src/main/java/org/apache/jackrabbit/core/data/GarbageCollector.java	(working copy)
@@ -18,6 +18,7 @@
 
 import org.apache.jackrabbit.api.management.DataStoreGarbageCollector;
 import org.apache.jackrabbit.api.management.MarkEventListener;
+import org.apache.jackrabbit.core.RepositoryContext;
 import org.apache.jackrabbit.core.SessionImpl;
 import org.apache.jackrabbit.core.id.NodeId;
 import org.apache.jackrabbit.core.id.PropertyId;
@@ -96,6 +97,8 @@
     private final SessionImpl[] sessionList;
 
     private final AtomicBoolean closed = new AtomicBoolean();
+    
+    private final RepositoryContext context;
 
     private boolean persistenceManagerScan;
 
@@ -105,14 +108,17 @@
      * Create a new garbage collector.
      * This method is usually not called by the application, it is called
      * by SessionImpl.createDataStoreGarbageCollector().
-     *
+     * 
+     * @param context repository context
      * @param dataStore the data store to be garbage-collected
      * @param list the persistence managers
      * @param sessionList the sessions to access the workspaces
      */
-    public GarbageCollector(
+    
+    public GarbageCollector( RepositoryContext context, 
             DataStore dataStore, IterablePersistenceManager[] list,
             SessionImpl[] sessionList) {
+        this.context = context;
         this.store = dataStore;
         this.pmList = list;
         this.persistenceManagerScan = list != null;
@@ -232,9 +238,13 @@
     }
 
     /**
-     * Stop the observation listener if any are installed.
+     * Reset modifiedDateOnAccess to 0 and stop the observation 
+     * listener if any are installed.
      */
     public void stopScan() throws RepositoryException {
+         // reset updateModifiedDateOnAccess to OL
+        store.updateModifiedDateOnAccess(0L);
+        
         if (listeners.size() > 0) {
             for (Listener listener : listeners) {
                 listener.stop();
@@ -242,6 +252,7 @@
             listeners.clear();
         }
         checkObservationException();
+        context.setGcRunning(false);
     }
 
     /**
Index: jackrabbit-core/src/main/java/org/apache/jackrabbit/core/RepositoryContext.java
===================================================================
--- jackrabbit-core/src/main/java/org/apache/jackrabbit/core/RepositoryContext.java	(revision 1461410)
+++ jackrabbit-core/src/main/java/org/apache/jackrabbit/core/RepositoryContext.java	(working copy)
@@ -122,6 +122,12 @@
      * The Statistics manager, handles statistics
      */
     private StatManager statManager;
+    
+    /**
+     *  flag to indicate if GC is running
+     */
+    
+    private boolean gcRunning;
 
     /**
      * Creates a component context for the given repository.
@@ -429,4 +435,22 @@
         return statManager;
     }
 
+    /**
+     * 
+     * @return gcRunning status
+     */
+    public boolean isGcRunning() {
+        return gcRunning;
+    }
+
+    /**
+     * set gcRunnign status
+     * @param gcRunning
+     */
+    public synchronized void setGcRunning(boolean gcRunning) {
+        this.gcRunning = gcRunning;
+    }
+    
+    
+
 }
Index: jackrabbit-core/src/main/java/org/apache/jackrabbit/core/RepositoryImpl.java
===================================================================
--- jackrabbit-core/src/main/java/org/apache/jackrabbit/core/RepositoryImpl.java	(revision 1461410)
+++ jackrabbit-core/src/main/java/org/apache/jackrabbit/core/RepositoryImpl.java	(working copy)
@@ -1422,7 +1422,14 @@
             }
             ipmList[i] = (IterablePersistenceManager) pm;
         }
-        return new GarbageCollector(context.getDataStore(), ipmList, sessions);
+        GarbageCollector gc = new GarbageCollector(context, context.getDataStore(), ipmList, sessions);
+        synchronized (this) {
+            if (context.isGcRunning()) {
+                throw new RepositoryException("Cannot create GC. GC already running");
+            }
+            context.setGcRunning(true);
+        }
+        return gc;
     }
 
     //-----------------------------------------------------------< Repository >
Index: jackrabbit-core/src/test/java/org/apache/jackrabbit/core/data/GarbageCollectorTest.java
===================================================================
--- jackrabbit-core/src/test/java/org/apache/jackrabbit/core/data/GarbageCollectorTest.java	(revision 1461410)
+++ jackrabbit-core/src/test/java/org/apache/jackrabbit/core/data/GarbageCollectorTest.java	(working copy)
@@ -169,7 +169,36 @@
 
         gc.close();
     }
+    
+    /**
+     *  Test to validate that two  GC cannot run simulatenously. one 
+     *  exits throwing exception  
+     */
+    public void testSimulatenousRunGC() throws Exception {
+        Node root = testRootNode;
+        Session session = root.getSession();
 
+        GCThread gct1 = new GCThread(session);
+        GCThread gct2 = new GCThread(session);
+        Thread gcThread1 = new Thread(gct1, "Datastore Garbage Collector 1");
+        Thread gcThread2 = new Thread(gct2, "Datastore Garbage Collector 2");
+        // run simulatensou gc
+        gcThread1.start();
+        gcThread2.start();
+        Thread.sleep(100);
+        
+        gct1.setStop(true);
+        gct2.setStop(true);
+        
+        // allow them to complete
+        gcThread1.join();
+        gcThread2.join();
+
+        // only one should throw error
+        int count = (gct1.getException() == null ? 0 : 1) + (gct2.getException() == null ? 0 : 1);
+        assertEquals("only one gc should throw exception ", 1, count);
+    }
+
     private void runGC(Session session, boolean all) throws Exception {
         GarbageCollector gc = ((SessionImpl)session).createDataStoreGarbageCollector();
         gc.setMarkEventListener(this);