[SPARK-6313] Add config option to disable file locks/fetchFile cache to ...

...support NFS mounts.

This is a work around for now with the goal to find a more permanent solution.
https://issues.apache.org/jira/browse/SPARK-6313

Author: nemccarthy <[email protected]>

Closes apache#5036 from nemccarthy/master and squashes the following commits:

2eaaf42 [nemccarthy] [SPARK-6313] Update config wording doc for spark.files.useFetchCache
5de7eb4 [nemccarthy] [SPARK-6313] Add config option to disable file locks/fetchFile cache to support NFS mounts

Author: nemccarthy

core/src/main/scala/org/apache/spark/util/Utils.scala

@@ -403,7 +403,8 @@ private[spark] object Utils extends Logging {
       useCache: Boolean) {
     val fileName = url.split("/").last
     val targetFile = new File(targetDir, fileName)
-    if (useCache) {
+    val fetchCacheEnabled = conf.getBoolean("spark.files.useFetchCache", defaultValue = true)
+    if (useCache && fetchCacheEnabled) {
       val cachedFileName = s"${url.hashCode}${timestamp}_cache"
       val lockFileName = s"${url.hashCode}${timestamp}_lock"
       val localDir = new File(getLocalDir(conf))

docs/configuration.md

@@ -745,6 +745,18 @@ Apart from these, the following properties are also available, and may be useful
     the driver, in seconds.
   </td>
 </tr>
+<tr>
+  <td><code>spark.files.useFetchCache</code></td>
+  <td>true</td>
+  <td>
+    If set to true (default), file fetching will use a local cache that is shared by executors
+    that belong to the same application, which can improve task launching performance when
+    running many executors on the same host. If set to false, these caching optimizations will
+    be disabled and all executors will fetch their own copies of files. This optimization may be
+    disabled in order to use Spark local directories that reside on NFS filesystems (see
+    <a href="https://issues.apache.org/jira/browse/SPARK-6313">SPARK-6313</a> for more details).
+  </td>
+</tr>
 <tr>
   <td><code>spark.files.overwrite</code></td>
   <td>false</td>