HADOOP-18254. Disable S3A prefetching by default. (apache#4469)

Contributed by Ahmar Suhail <[email protected]>

Author: ahmarsuhail

hadoop-tools/hadoop-aws/src/main/java/org/apache/hadoop/fs/s3a/S3AFileSystem.java

@@ -504,7 +504,7 @@ public void initialize(URI name, Configuration originalConf)
       longBytesOption(conf, FS_S3A_BLOCK_SIZE, DEFAULT_BLOCKSIZE, 1);
       enableMultiObjectsDelete = conf.getBoolean(ENABLE_MULTI_DELETE, true);
 
-      this.prefetchEnabled = conf.getBoolean(PREFETCH_ENABLED_KEY, true);
+      this.prefetchEnabled = conf.getBoolean(PREFETCH_ENABLED_KEY, false);
       this.prefetchBlockSize = intOption(
           conf, PREFETCH_BLOCK_SIZE_KEY, PREFETCH_BLOCK_DEFAULT_SIZE, PREFETCH_BLOCK_DEFAULT_SIZE);
       this.prefetchBlockCount =

hadoop-tools/hadoop-aws/src/main/java/org/apache/hadoop/fs/s3a/read/README.md

@@ -47,6 +47,7 @@ full details.
 * [Auditing](./auditing.html).
 * [Auditing Architecture](./auditing_architecture.html).
 * [Testing](./testing.html)
+* [Prefetching](./prefetching.html)
 
 ## <a name="overview"></a> Overview
 
@@ -1090,6 +1091,29 @@ options are covered in [Testing](./testing.md).
   </description>
 </property>
 
+<property>
+  <name>fs.s3a.prefetch.enabled</name>
+  <value>false</value>
+  <description>
+    Enables prefetching and caching when reading from input stream.
+  </description>
+</property>
+
+<property>
+  <name>fs.s3a.prefetch.block.size</name>
+  <value>8MB</value>
+  <description>
+      The size of a single prefetched block of data.
+  </description>
+</property>
+
+<property>
+  <name>fs.s3a.prefetch.block.count</name>
+  <value>8</value>
+  <description>
+      Maximum number of blocks prefetched concurrently at any given time.
+  </description>
+</property>
 ```
 
 ## <a name="retry_and_recovery"></a>Retry and Recovery

hadoop-tools/hadoop-aws/src/site/markdown/tools/hadoop-aws/index.md

@@ -39,7 +39,7 @@ Multiple blocks may be read in parallel.
 
 |Property    |Meaning    |Default    |
 |---|---|---|
-|`fs.s3a.prefetch.enabled`    |Enable the prefetch input stream    |`true` |
+|`fs.s3a.prefetch.enabled`    |Enable the prefetch input stream    |`false` |
 |`fs.s3a.prefetch.block.size`    |Size of a block    |`8M`    |
 |`fs.s3a.prefetch.block.count`    |Number of blocks to prefetch    |`8`    |