[SPARK-9934] Deprecate NIO ConnectionManager.

Deprecate NIO ConnectionManager in Spark 1.5.0, before removing it in Spark 1.6.0.

Author: Reynold Xin <rxin@databricks.com>

Closes #8162 from rxin/SPARK-9934.

core/src/main/scala/org/apache/spark/SparkEnv.scala

@@ -331,6 +331,8 @@ object SparkEnv extends Logging {
         case "netty" =>
           new NettyBlockTransferService(conf, securityManager, numUsableCores)
         case "nio" =>
+          logWarning("NIO-based block transfer service is deprecated, " +
+            "and will be removed in Spark 1.6.0.")
           new NioBlockTransferService(conf, securityManager)
       }
 

docs/configuration.md

@@ -389,7 +389,8 @@ Apart from these, the following properties are also available, and may be useful
     Implementation to use for transferring shuffle and cached blocks between executors. There
     are two implementations available: <code>netty</code> and <code>nio</code>. Netty-based
     block transfer is intended to be simpler but equally efficient and is the default option
-    starting in 1.2.
+    starting in 1.2, and <code>nio</code> block transfer is deprecated in Spark 1.5.0 and will
+    be removed in Spark 1.6.0.
   </td>
 </tr>
 <tr>