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

HDFS Short-Circuit Local Reads

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%{toc|section=1|fromDepth=0}

* {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.

* {Configuration}

  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 DataNodes 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.

  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>
----

* {Configuration Keys}

  * dfs.client.read.shortcircuit

  This configuration parameter turns on short-circuit local reads.

  * dfs.client.read.shortcircuit.skip.checkusm

  If this configuration parameter is set, short-circuit local reads will skip
  checksums.  This is normally not recommended, but it may be useful for
  special setups.  You might consider using this if you are doing your own
  checksumming outside of HDFS.

  * dfs.client.read.shortcircuit.streams.cache.size

  The DFSClient maintains a cache of recently opened file descriptors.  This
  parameter controls the size of that cache.  Setting this higher will use more
  file descriptors, but potentially provide better performance on workloads
  involving lots of seeks.

  * dfs.client.read.shortcircuit.streams.cache.expiry.ms

  This controls the minimum amount of time file descriptors need to sit in the
  FileInputStreamCache before they can be closed for being inactive for too long.

  * dfs.client.domain.socket.data.traffic

  This control whether we will try to pass normal data traffic over UNIX domain
  sockets.