make tachyonStore in BlockMananger lazy val; add more comments StorageLevels.

Author: haoyuan

core/src/main/java/org/apache/spark/api/java/StorageLevels.java

@@ -37,6 +37,14 @@ public class StorageLevels {
 
   public static final StorageLevel TACHYON = new StorageLevel(false, false, true, false, 1);
 
+  /**
+   * Create a new StorageLevel object.
+   * @param useDisk saved to disk, if true
+   * @param useMemory saved to memory, if true
+   * @param deserialized saved as deserialized objects, if true
+   * @param replication replication factor
+   */
+  @Deprecated
   public static StorageLevel create(boolean useDisk, boolean useMemory, boolean deserialized, int replication) {
     return StorageLevel.apply(useDisk, useMemory, false, deserialized, replication);
   }
@@ -45,6 +53,7 @@ public static StorageLevel create(boolean useDisk, boolean useMemory, boolean de
    * Create a new StorageLevel object.
    * @param useDisk saved to disk, if true
    * @param useMemory saved to memory, if true
+   * @param useTachyon saved to Tachyon, if true
    * @param deserialized saved as deserialized objects, if true
    * @param replication replication factor
    */

core/src/main/scala/org/apache/spark/storage/BlockManager.scala

@@ -101,7 +101,7 @@ private[spark] class BlockManager(
   var asyncReregisterTask: Future[Unit] = null
   val asyncReregisterLock = new Object
 
-  private def tachyonStore : TachyonStore = synchronized {
+  private[storage] lazy val tachyonStore : TachyonStore = {
    if (!tachyonInitialized) {
      initializeTachyonStore() 
    }