hadoop-hdfs-project/hadoop-hdfs/src/site/apt/ShortCircuitLocalReads.apt.vm

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  Hadoop Distributed File System-${project.version} - Short-Circuit Local Reads
  ---
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  ${maven.build.timestamp}

HDFS Short-Circuit Local Reads

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* {Short-Circuit Local Reads}

** Background

  In <<<HDFS>>>, reads normally go through the <<<DataNode>>>.  Thus, when the
  client asks the <<<DataNode>>> to read a file, the <<<DataNode>>> reads that
  file off of the disk and sends the data to the client over a TCP socket.
  So-called "short-circuit" reads bypass the <<<DataNode>>>, allowing the client
  to read the file directly.  Obviously, this is only possible in cases where
  the client is co-located with the data.  Short-circuit reads provide a
  substantial performance boost to many applications.

** Setup

  To configure short-circuit local reads, you will need to enable
  <<<libhadoop.so>>>.  See
  {{{../hadoop-common/NativeLibraries.html}Native
  Libraries}} for details on enabling this library.

  Short-circuit reads make use of a UNIX domain socket.  This is a special path
  in the filesystem that allows the client and the <<<DataNode>>>s to communicate.
  You will need to set a path to this socket.  The <<<DataNode>>> needs to be able to
  create this path.  On the other hand, it should not be possible for any user
  except the HDFS user or root to create this path.  For this reason, paths
  under <<</var/run>>> or <<</var/lib>>> are often used.

  The client and the <<<DataNode>>> exchange information via a shared memory segment
  on <<</dev/shm>>>.

  Short-circuit local reads need to be configured on both the <<<DataNode>>>
  and the client.

** Example Configuration

  Here is an example configuration.

----
<configuration>
  <property>
    <name>dfs.client.read.shortcircuit</name>
    <value>true</value>
  </property>
  <property>
    <name>dfs.domain.socket.path</name>
    <value>/var/lib/hadoop-hdfs/dn_socket</value>
  </property>
</configuration>
----

* Legacy HDFS Short-Circuit Local Reads

  Legacy implementation of short-circuit local reads
  on which the clients directly open the HDFS block files
  is still available for platforms other than the Linux.
  Setting the value of <<<dfs.client.use.legacy.blockreader.local>>>
  in addition to <<<dfs.client.read.shortcircuit>>>
  to true enables this feature.

  You also need to set the value of <<<dfs.datanode.data.dir.perm>>>
  to <<<750>>> instead of the default <<<700>>> and
  chmod/chown the directory tree under <<<dfs.datanode.data.dir>>>
  as readable to the client and the <<<DataNode>>>.
  You must take caution because this means that
  the client can read all of the block files bypassing HDFS permission.

  Because Legacy short-circuit local reads is insecure,
  access to this feature is limited to the users listed in
  the value of <<<dfs.block.local-path-access.user>>>.

----
<configuration>
  <property>
    <name>dfs.client.read.shortcircuit</name>
    <value>true</value>
  </property>
  <property>
    <name>dfs.client.use.legacy.blockreader.local</name>
    <value>true</value>
  </property>
  <property>
    <name>dfs.datanode.data.dir.perm</name>
    <value>750</value>
  </property>
  <property>
    <name>dfs.block.local-path-access.user</name>
    <value>foo,bar</value>
  </property>
</configuration>
----