HDFS-3697. Enable fadvise readahead by default. Contributed by Todd Lipcon.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/trunk@1364698 13f79535-47bb-0310-9956-ffa450edef68

hadoop-hdfs-project/hadoop-hdfs/CHANGES.txt

@@ -363,6 +363,8 @@ Branch-2 ( Unreleased changes )
     HDFS-3110. Use directRead API to reduce the number of buffer copies in
     libhdfs (Henry Robinson via todd)
 
+    HDFS-3697. Enable fadvise readahead by default. (todd)
+
   BUG FIXES
 
     HDFS-3385. The last block of INodeFileUnderConstruction is not

hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/DFSConfigKeys.java

@@ -74,7 +74,7 @@ public class DFSConfigKeys extends CommonConfigurationKeys {
   public static final String  DFS_DATANODE_BALANCE_BANDWIDTHPERSEC_KEY = "dfs.datanode.balance.bandwidthPerSec";
   public static final long    DFS_DATANODE_BALANCE_BANDWIDTHPERSEC_DEFAULT = 1024*1024;
   public static final String  DFS_DATANODE_READAHEAD_BYTES_KEY = "dfs.datanode.readahead.bytes";
-  public static final long    DFS_DATANODE_READAHEAD_BYTES_DEFAULT = 0;
+  public static final long    DFS_DATANODE_READAHEAD_BYTES_DEFAULT = 4 * 1024 * 1024; // 4MB
   public static final String  DFS_DATANODE_DROP_CACHE_BEHIND_WRITES_KEY = "dfs.datanode.drop.cache.behind.writes";
   public static final boolean DFS_DATANODE_DROP_CACHE_BEHIND_WRITES_DEFAULT = false;
   public static final String  DFS_DATANODE_SYNC_BEHIND_WRITES_KEY = "dfs.datanode.sync.behind.writes";

hadoop-hdfs-project/hadoop-hdfs/src/main/resources/hdfs-default.xml

@@ -714,6 +714,80 @@
   </description>
 </property>
 
+<property>
+  <name>dfs.datanode.readahead.bytes</name>
+  <value>4193404</value>
+  <description>
+        While reading block files, if the Hadoop native libraries are available,
+        the datanode can use the posix_fadvise system call to explicitly
+        page data into the operating system buffer cache ahead of the current
+        reader's position. This can improve performance especially when
+        disks are highly contended.
+
+        This configuration specifies the number of bytes ahead of the current
+        read position which the datanode will attempt to read ahead. This
+        feature may be disabled by configuring this property to 0.
+
+        If the native libraries are not available, this configuration has no
+        effect.
+  </description>
+</property>
+
+<property>
+  <name>dfs.datanode.drop.cache.behind.reads</name>
+  <value>false</value>
+  <description>
+        In some workloads, the data read from HDFS is known to be significantly
+        large enough that it is unlikely to be useful to cache it in the
+        operating system buffer cache. In this case, the DataNode may be
+        configured to automatically purge all data from the buffer cache
+        after it is delivered to the client. This behavior is automatically
+        disabled for workloads which read only short sections of a block
+        (e.g HBase random-IO workloads).
+
+        This may improve performance for some workloads by freeing buffer
+        cache spage usage for more cacheable data.
+
+        If the Hadoop native libraries are not available, this configuration
+        has no effect.
+  </description>
+</property>
+
+<property>
+  <name>dfs.datanode.drop.cache.behind.writes</name>
+  <value>false</value>
+  <description>
+        In some workloads, the data written to HDFS is known to be significantly
+        large enough that it is unlikely to be useful to cache it in the
+        operating system buffer cache. In this case, the DataNode may be
+        configured to automatically purge all data from the buffer cache
+        after it is written to disk.
+
+        This may improve performance for some workloads by freeing buffer
+        cache spage usage for more cacheable data.
+
+        If the Hadoop native libraries are not available, this configuration
+        has no effect.
+  </description>
+</property>
+
+<property>
+  <name>dfs.datanode.sync.behind.writes</name>
+  <value>false</value>
+  <description>
+        If this configuration is enabled, the datanode will instruct the
+        operating system to enqueue all written data to the disk immediately
+        after it is written. This differs from the usual OS policy which
+        may wait for up to 30 seconds before triggering writeback.
+
+        This may improve performance for some workloads by smoothing the
+        IO profile for data written to disk.
+
+        If the Hadoop native libraries are not available, this configuration
+        has no effect.
+  </description>
+</property>
+
 <property>
   <name>dfs.client.failover.max.attempts</name>
   <value>15</value>