Homec4science
Diffusion elastic-yarn (master) Browse

R3704/2.3.0/hadoop-2.3.0/share/hadoop/hadoop-project-dist/hadoop-hdfs/ShortCircuitLocalReads.html
6c2cef95801emaster

Last Change
Raw File

ShortCircuitLocalReads.html
View Options

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<!-- Generated by Apache Maven Doxia at 2014-02-11 -->
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<title>Apache Hadoop 2.3.0 - Hadoop Distributed File System-2.3.0 - Short-Circuit Local Reads</title>
<style type="text/css" media="all">
@import url("./css/maven-base.css");
@import url("./css/maven-theme.css");
@import url("./css/site.css");
</style>
<link rel="stylesheet" href="./css/print.css" type="text/css" media="print" />
<meta name="Date-Revision-yyyymmdd" content="20140211" />
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
</head>
<body class="composite">
<div id="banner">
<a href="http://hadoop.apache.org/" id="bannerLeft">
<img src="http://hadoop.apache.org/images/hadoop-logo.jpg" alt="" />
</a>
<a href="http://www.apache.org/" id="bannerRight">
<img src="http://www.apache.org/images/asf_logo_wide.png" alt="" />
</a>
<div class="clear">
<hr/>
</div>
</div>
<div id="breadcrumbs">
<div class="xleft">
<a href="http://www.apache.org/" class="externalLink">Apache</a>
&gt;
<a href="http://hadoop.apache.org/" class="externalLink">Hadoop</a>
&gt;
<a href="../">Apache Hadoop Project Dist POM</a>
&gt;
Apache Hadoop 2.3.0
</div>
<div class="xright"> <a href="http://wiki.apache.org/hadoop" class="externalLink">Wiki</a>
|
<a href="https://svn.apache.org/repos/asf/hadoop/" class="externalLink">SVN</a>
|
<a href="http://hadoop.apache.org/" class="externalLink">Apache Hadoop</a>
&nbsp;| Last Published: 2014-02-11
&nbsp;| Version: 2.3.0
</div>
<div class="clear">
<hr/>
</div>
</div>
<div id="leftColumn">
<div id="navcolumn">
<h5>General</h5>
<ul>
<li class="none">
<a href="../../index.html">Overview</a>
</li>
<li class="none">
<a href="../../hadoop-project-dist/hadoop-common/SingleCluster.html">Single Node Setup</a>
</li>
<li class="none">
<a href="../../hadoop-project-dist/hadoop-common/ClusterSetup.html">Cluster Setup</a>
</li>
<li class="none">
<a href="../../hadoop-project-dist/hadoop-common/CommandsManual.html">Hadoop Commands Reference</a>
</li>
<li class="none">
<a href="../../hadoop-project-dist/hadoop-common/FileSystemShell.html">File System Shell</a>
</li>
<li class="none">
<a href="../../hadoop-project-dist/hadoop-common/Compatibility.html">Hadoop Compatibility</a>
</li>
</ul>
<h5>Common</h5>
<ul>
<li class="none">
<a href="../../hadoop-project-dist/hadoop-common/CLIMiniCluster.html">CLI Mini Cluster</a>
</li>
<li class="none">
<a href="../../hadoop-project-dist/hadoop-common/NativeLibraries.html">Native Libraries</a>
</li>
<li class="none">
<a href="../../hadoop-project-dist/hadoop-common/Superusers.html">Superusers</a>
</li>
<li class="none">
<a href="../../hadoop-project-dist/hadoop-common/SecureMode.html">Secure Mode</a>
</li>
<li class="none">
<a href="../../hadoop-project-dist/hadoop-common/ServiceLevelAuth.html">Service Level Authorization</a>
</li>
<li class="none">
<a href="../../hadoop-project-dist/hadoop-common/HttpAuthentication.html">HTTP Authentication</a>
</li>
</ul>
<h5>HDFS</h5>
<ul>
<li class="none">
<a href="../../hadoop-project-dist/hadoop-hdfs/HdfsUserGuide.html">HDFS User Guide</a>
</li>
<li class="none">
<a href="../../hadoop-yarn/hadoop-yarn-site/HDFSHighAvailabilityWithQJM.html">High Availability With QJM</a>
</li>
<li class="none">
<a href="../../hadoop-yarn/hadoop-yarn-site/HDFSHighAvailabilityWithNFS.html">High Availability With NFS</a>
</li>
<li class="none">
<a href="../../hadoop-project-dist/hadoop-hdfs/Federation.html">Federation</a>
</li>
<li class="none">
<a href="../../hadoop-project-dist/hadoop-hdfs/HdfsSnapshots.html">HDFS Snapshots</a>
</li>
<li class="none">
<a href="../../hadoop-project-dist/hadoop-hdfs/HdfsDesign.html">HDFS Architecture</a>
</li>
<li class="none">
<a href="../../hadoop-project-dist/hadoop-hdfs/HdfsEditsViewer.html">Edits Viewer</a>
</li>
<li class="none">
<a href="../../hadoop-project-dist/hadoop-hdfs/HdfsImageViewer.html">Image Viewer</a>
</li>
<li class="none">
<a href="../../hadoop-project-dist/hadoop-hdfs/HdfsPermissionsGuide.html">Permissions and HDFS</a>
</li>
<li class="none">
<a href="../../hadoop-project-dist/hadoop-hdfs/HdfsQuotaAdminGuide.html">Quotas and HDFS</a>
</li>
<li class="none">
<a href="../../hadoop-project-dist/hadoop-hdfs/Hftp.html">HFTP</a>
</li>
<li class="none">
<a href="../../hadoop-project-dist/hadoop-hdfs/LibHdfs.html">C API libhdfs</a>
</li>
<li class="none">
<a href="../../hadoop-project-dist/hadoop-hdfs/WebHDFS.html">WebHDFS REST API</a>
</li>
<li class="none">
<a href="../../hadoop-hdfs-httpfs/index.html">HttpFS Gateway</a>
</li>
<li class="none">
<a href="../../hadoop-project-dist/hadoop-hdfs/ShortCircuitLocalReads.html">Short Circuit Local Reads</a>
</li>
<li class="none">
<a href="../../hadoop-project-dist/hadoop-hdfs/CentralizedCacheManagement.html">Centralized Cache Management</a>
</li>
<li class="none">
<a href="../../hadoop-project-dist/hadoop-hdfs/HdfsNfsGateway.html">HDFS NFS Gateway</a>
</li>
</ul>
<h5>MapReduce</h5>
<ul>
<li class="none">
<a href="../../hadoop-mapreduce-client/hadoop-mapreduce-client-core/MapReduce_Compatibility_Hadoop1_Hadoop2.html">Compatibilty between Hadoop 1.x and Hadoop 2.x</a>
</li>
<li class="none">
<a href="../../hadoop-mapreduce-client/hadoop-mapreduce-client-core/EncryptedShuffle.html">Encrypted Shuffle</a>
</li>
<li class="none">
<a href="../../hadoop-mapreduce-client/hadoop-mapreduce-client-core/PluggableShuffleAndPluggableSort.html">Pluggable Shuffle/Sort</a>
</li>
<li class="none">
<a href="../../hadoop-mapreduce-client/hadoop-mapreduce-client-core/DistributedCacheDeploy.html">Distributed Cache Deploy</a>
</li>
</ul>
<h5>YARN</h5>
<ul>
<li class="none">
<a href="../../hadoop-yarn/hadoop-yarn-site/YARN.html">YARN Architecture</a>
</li>
<li class="none">
<a href="../../hadoop-yarn/hadoop-yarn-site/WritingYarnApplications.html">Writing YARN Applications</a>
</li>
<li class="none">
<a href="../../hadoop-yarn/hadoop-yarn-site/CapacityScheduler.html">Capacity Scheduler</a>
</li>
<li class="none">
<a href="../../hadoop-yarn/hadoop-yarn-site/FairScheduler.html">Fair Scheduler</a>
</li>
<li class="none">
<a href="../../hadoop-yarn/hadoop-yarn-site/WebApplicationProxy.html">Web Application Proxy</a>
</li>
<li class="none">
<a href="../../hadoop-yarn/hadoop-yarn-site/YarnCommands.html">YARN Commands</a>
</li>
<li class="none">
<a href="../../hadoop-sls/SchedulerLoadSimulator.html">Scheduler Load Simulator</a>
</li>
</ul>
<h5>YARN REST APIs</h5>
<ul>
<li class="none">
<a href="../../hadoop-yarn/hadoop-yarn-site/WebServicesIntro.html">Introduction</a>
</li>
<li class="none">
<a href="../../hadoop-yarn/hadoop-yarn-site/ResourceManagerRest.html">Resource Manager</a>
</li>
<li class="none">
<a href="../../hadoop-yarn/hadoop-yarn-site/NodeManagerRest.html">Node Manager</a>
</li>
<li class="none">
<a href="../../hadoop-yarn/hadoop-yarn-site/MapredAppMasterRest.html">MR Application Master</a>
</li>
<li class="none">
<a href="../../hadoop-yarn/hadoop-yarn-site/HistoryServerRest.html">History Server</a>
</li>
</ul>
<h5>Auth</h5>
<ul>
<li class="none">
<a href="../../hadoop-auth/index.html">Overview</a>
</li>
<li class="none">
<a href="../../hadoop-auth/Examples.html">Examples</a>
</li>
<li class="none">
<a href="../../hadoop-auth/Configuration.html">Configuration</a>
</li>
<li class="none">
<a href="../../hadoop-auth/BuildingIt.html">Building</a>
</li>
</ul>
<h5>Reference</h5>
<ul>
<li class="none">
<a href="../../hadoop-project-dist/hadoop-common/releasenotes.html">Release Notes</a>
</li>
<li class="none">
<a href="../../api/index.html">API docs</a>
</li>
<li class="none">
<a href="../../hadoop-project-dist/hadoop-common/CHANGES.txt">Common CHANGES.txt</a>
</li>
<li class="none">
<a href="../../hadoop-project-dist/hadoop-hdfs/CHANGES.txt">HDFS CHANGES.txt</a>
</li>
<li class="none">
<a href="../../hadoop-project-dist/hadoop-mapreduce/CHANGES.txt">MapReduce CHANGES.txt</a>
</li>
</ul>
<h5>Configuration</h5>
<ul>
<li class="none">
<a href="../../hadoop-project-dist/hadoop-common/core-default.xml">core-default.xml</a>
</li>
<li class="none">
<a href="../../hadoop-project-dist/hadoop-hdfs/hdfs-default.xml">hdfs-default.xml</a>
</li>
<li class="none">
<a href="../../hadoop-mapreduce-client/hadoop-mapreduce-client-core/mapred-default.xml">mapred-default.xml</a>
</li>
<li class="none">
<a href="../../hadoop-yarn/hadoop-yarn-common/yarn-default.xml">yarn-default.xml</a>
</li>
<li class="none">
<a href="../../hadoop-project-dist/hadoop-common/DeprecatedProperties.html">Deprecated Properties</a>
</li>
</ul>
<a href="http://maven.apache.org/" title="Built by Maven" class="poweredBy">
<img alt="Built by Maven" src="./images/logos/maven-feather.png"/>
</a>
</div>
</div>
<div id="bodyColumn">
<div id="contentBox">
<!-- Licensed under the Apache License, Version 2.0 (the "License"); --><!-- you may not use this file except in compliance with the License. --><!-- You may obtain a copy of the License at --><!-- --><!-- http://www.apache.org/licenses/LICENSE-2.0 --><!-- --><!-- Unless required by applicable law or agreed to in writing, software --><!-- distributed under the License is distributed on an "AS IS" BASIS, --><!-- WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. --><!-- See the License for the specific language governing permissions and --><!-- limitations under the License. See accompanying LICENSE file. --><div class="section">
<h2>HDFS Short-Circuit Local Reads<a name="HDFS_Short-Circuit_Local_Reads"></a></h2>
<ul>
<li><a href="#HDFS_Short-Circuit_Local_Reads">HDFS Short-Circuit Local Reads</a>
<ul>
<li><a href="#Background">Background</a></li>
<li><a href="#Configuration">Configuration</a></li>
<li><a href="#Example_Configuration">Example Configuration</a></li>
<li><a href="#Configuration_Keys">Configuration Keys</a></li></ul></li></ul>
<div class="section">
<h3><a name="Background">Background</a></h3>
<p>In <tt>HDFS</tt>, reads normally go through the <tt>DataNode</tt>. Thus, when the client asks the <tt>DataNode</tt> to read a file, the <tt>DataNode</tt> reads that file off of the disk and sends the data to the client over a TCP socket. So-called &quot;short-circuit&quot; reads bypass the <tt>DataNode</tt>, 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.</p></div>
<div class="section">
<h3><a name="Configuration">Configuration</a></h3>
<p>To configure short-circuit local reads, you will need to enable <tt>libhadoop.so</tt>. See <a href="../hadoop-common/NativeLibraries.html">Native Libraries</a> for details on enabling this library.</p>
<p>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 <tt>/var/run</tt> or <tt>/var/lib</tt> are often used.</p>
<p>Short-circuit local reads need to be configured on both the <tt>DataNode</tt> and the client.</p></div>
<div class="section">
<h3><a name="Example_Configuration">Example Configuration</a></h3>
<p>Here is an example configuration.</p>
<div>
<pre>&lt;configuration&gt;
&lt;property&gt;
&lt;name&gt;dfs.client.read.shortcircuit&lt;/name&gt;
&lt;value&gt;true&lt;/value&gt;
&lt;/property&gt;
&lt;property&gt;
&lt;name&gt;dfs.domain.socket.path&lt;/name&gt;
&lt;value&gt;/var/lib/hadoop-hdfs/dn_socket&lt;/value&gt;
&lt;/property&gt;
&lt;/configuration&gt;</pre></div></div>
<div class="section">
<h3><a name="Configuration_Keys">Configuration Keys</a></h3>
<ul>
<li>dfs.client.read.shortcircuit
<p>This configuration parameter turns on short-circuit local reads.</p></li>
<li>dfs.client.read.shortcircuit.skip.checksum
<p>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.</p></li>
<li>dfs.client.read.shortcircuit.streams.cache.size
<p>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.</p></li>
<li>dfs.client.read.shortcircuit.streams.cache.expiry.ms
<p>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.</p></li>
<li>dfs.client.domain.socket.data.traffic
<p>This control whether we will try to pass normal data traffic over UNIX domain sockets.</p></li></ul></div></div>
</div>
</div>
<div class="clear">
<hr/>
</div>
<div id="footer">
<div class="xright">&#169; 2014
Apache Software Foundation
- <a href="http://maven.apache.org/privacy-policy.html">Privacy Policy</a></div>
<div class="clear">
<hr/>
</div>
</div>
</body>
</html>
c4science · Help