Index: MongoBlobStore.java
===================================================================
--- MongoBlobStore.java	(revision 1614633)
+++ MongoBlobStore.java	(working copy)
@@ -165,6 +165,10 @@
         DBObject options = new BasicDBObject();
         options.put("unique", Boolean.TRUE);
         collection.ensureIndex(index, options);
+        // index on the lastMod to speed-up GC queries
+        index = new BasicDBObject();
+        index.put(MongoBlob.KEY_LAST_MOD, 1L);
+        collection.ensureIndex(index);
     }
 
     private MongoBlob getBlob(String id, long lastMod) {
@@ -228,8 +232,7 @@
         }
 
         final DBCursor cur =
-                collection.find(builder.get(), fields).hint(fields)
-                        .addOption(Bytes.QUERYOPTION_SLAVEOK);
+                collection.find(builder.get(), fields).addOption(Bytes.QUERYOPTION_SLAVEOK);
 
         //TODO The cursor needs to be closed
         return new AbstractIterator<String>() {