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+ PDF</a>
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+</div>
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+<h1>
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+ Hadoop DFS User Guide
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+ </h1>
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+<div id="minitoc-area">
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+<ul class="minitoc">
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+<li>
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+<a href="#Purpose">Purpose</a>
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+</li>
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+<li>
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+<a href="#Overview"> Overview </a>
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+</li>
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+<li>
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+<a href="#Pre-requisites"> Pre-requisites </a>
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+</li>
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+<li>
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+<a href="#Web+Interface"> Web Interface </a>
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+</li>
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+<li>
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+<a href="#Shell+Commands">Shell Commands</a>
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+<ul class="minitoc">
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+<li>
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+<a href="#DFSAdmin+Command"> DFSAdmin Command </a>
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+</li>
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+</ul>
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+</li>
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+<li>
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+<a href="#Secondary+Namenode"> Secondary Namenode </a>
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+</li>
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+<li>
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+<a href="#Rebalancer"> Rebalancer </a>
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+</li>
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+<li>
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+<a href="#Rack+Awareness"> Rack Awareness </a>
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+</li>
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+<li>
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+<a href="#Safemode"> Safemode </a>
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+</li>
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+<li>
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+<a href="#Fsck"> Fsck </a>
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+</li>
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+<li>
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+<a href="#Upgrade+and+Rollback"> Upgrade and Rollback </a>
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+</li>
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+<li>
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+<a href="#File+Permissions+and+Security"> File Permissions and Security </a>
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+</li>
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+<li>
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+<a href="#Scalability"> Scalability </a>
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+</li>
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+<li>
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+<a href="#Related+Documentation"> Related Documentation </a>
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+</li>
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+</ul>
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+</div>
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+
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+<a name="N1000C"></a><a name="Purpose"></a>
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+<h2 class="h3">Purpose</h2>
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+<div class="section">
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+<p>
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+ This document aims to be the starting point for users working with
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+ Hadoop Distributed File System (HDFS) either as a part of a
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+ <a href="http://hadoop.apache.org/">Hadoop</a>
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+ cluster or as a stand-alone general purpose distributed file system.
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+ While HDFS is designed to "just-work" in many environments, a working
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+ knowledge of HDFS helps greatly with configuration improvements and
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+ diagnostics on a specific cluster.
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+ </p>
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+</div>
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+
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+
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+<a name="N1001A"></a><a name="Overview"></a>
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+<h2 class="h3"> Overview </h2>
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+<div class="section">
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+<p>
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+ HDFS is the primary distributed storage used by Hadoop applications. A
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+ HDFS cluster primarily consists of a <em>NameNode</em> that manages the
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+ filesystem metadata and Datanodes that store the actual data. The
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+ architecture of HDFS is described in detail
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+ <a href="hdfs_design.html">here</a>. This user guide primarily deals with
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+ interaction of users and administrators with HDFS clusters.
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+ The <a href="images/hdfsarchitecture.gif">diagram</a> from
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+ <a href="hdfs_design.html">HDFS architecture</a> depicts
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+ basic interactions among Namenode, Datanodes, and the clients. Eseentially,
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+ clients contact Namenode for file metadata or file modifications and perform
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+ actual file I/O directly with the datanodes.
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+ </p>
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+<p>
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+ The following are some of the salient features that could be of
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+ interest to many users. The terms in <em>italics</em>
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+ are described in later sections.
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+ </p>
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+<ul>
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+
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+<li>
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+ Hadoop, including HDFS, is well suited for distributed storage
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+ and distributed processing using commodity hardware. It is fault
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+ tolerant, scalable, and extremely simple to expand.
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+ <a href="mapred_tutorial.html">Map-Reduce</a>,
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+ well known for its simplicity and applicability for large set of
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+ distributed applications, is an integral part of Hadoop.
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+ </li>
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+
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+<li>
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+ HDFS is highly configurable with a default configuration well
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+ suited for many installations. Most of the time, configuration
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+ needs to be tuned only for very large clusters.
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+ </li>
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+
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+<li>
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+ It is written in Java and is supported on all major platforms.
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+ </li>
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+
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+<li>
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+ Supports <em>shell like commands</em> to interact with HDFS directly.
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+ </li>
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+
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+<li>
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+ Namenode and Datanodes have built in web servers that makes it
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+ easy to check current status of the cluster.
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+ </li>
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+
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+<li>
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+ New features and improvements are regularly implemented in HDFS.
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+ The following is a subset of useful features in HDFS:
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+ <ul>
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+
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+<li>
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+
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+<em>File permissions and authentication.</em>
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+
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+</li>
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+
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+<li>
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+
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+<em>Rack awareness</em> : to take a node's physical location into
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+ account while scheduling tasks and allocating storage.
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+ </li>
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+
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+<li>
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+
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+<em>Safemode</em> : an administrative mode for maintanance.
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+ </li>
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+
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+<li>
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+
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+<em>fsck</em> : an utility to diagnose health of the filesystem, to
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+ find missing files or blocks.
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+ </li>
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+
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+<li>
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+
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+<em>Rebalancer</em> : tool to balance the cluster when the data is
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+ unevenly distributed among datanodes.
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+ </li>
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+
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+<li>
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+
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+<em>Upgrade and Rollback</em> : after a software upgrade,
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+ it is possible to
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+ rollback to HDFS' state before the upgrade in case of unexpected
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+ problems.
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+ </li>
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+
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+<li>
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+
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+<em>Secondary Namenode</em> : helps keep the size of file
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+ containing log of HDFS modification with in certain limit at
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+ the Namenode.
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+ </li>
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+
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+</ul>
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+
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+</li>
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+
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+</ul>
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+</div>
|
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+<a name="N10082"></a><a name="Pre-requisites"></a>
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+<h2 class="h3"> Pre-requisites </h2>
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+<div class="section">
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+<p>
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+ The following documents describe installation and set up of a
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+ Hadoop cluster :
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+ </p>
|
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+<ul>
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+
|
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+<li>
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+
|
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+<a href="quickstart.html">Hadoop Quickstart</a>
|
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|
+ for first-time users.
|
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+ </li>
|
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+
|
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+<li>
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+
|
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+<a href="cluster_setup.html">Hadoop Cluster Setup</a>
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|
+ for large, distributed clusters.
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+ </li>
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+
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+</ul>
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+<p>
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+ The rest of document assumes the user is able to set up and run a
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+ HDFS with at least one Datanode. For the purpose of this document,
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+ both Namenode and Datanode could be running on the same physical
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+ machine.
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+ </p>
|
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+</div>
|
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+<a name="N100A0"></a><a name="Web+Interface"></a>
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+<h2 class="h3"> Web Interface </h2>
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+<div class="section">
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+<p>
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+ Namenode and Datanode each run an internal web server in order to
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+ display basic information about the current status of the cluster.
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+ With the default configuration, namenode front page is at
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+ <span class="codefrag">http://namenode:50070/</span> .
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+ It lists the datanodes in the cluster and basic stats of the
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+ cluster. The web interface can also be used to browse the file
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+ system (using "Browse the file system" link on the Namenode front
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+ page).
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+ </p>
|
|
|
+</div>
|
|
|
+<a name="N100AD"></a><a name="Shell+Commands"></a>
|
|
|
+<h2 class="h3">Shell Commands</h2>
|
|
|
+<div class="section">
|
|
|
+<p>
|
|
|
+ Hadoop includes various "shell-like" commands that directly
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|
+ interact with HDFS and other file systems that Hadoop supports.
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|
+ The command
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|
+ <span class="codefrag">bin/hadoop fs -help</span>
|
|
|
+ lists the commands supported by Hadoop
|
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+ shell. Further,
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|
+ <span class="codefrag">bin/hadoop fs -help command</span>
|
|
|
+ displays more detailed help on a command. The commands support
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+ most of the normal filesystem operations like copying files,
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+ changing file permissions, etc. It also supports a few HDFS
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+ specific operations like changing replication of files.
|
|
|
+ </p>
|
|
|
+<a name="N100BC"></a><a name="DFSAdmin+Command"></a>
|
|
|
+<h3 class="h4"> DFSAdmin Command </h3>
|
|
|
+<p>
|
|
|
+
|
|
|
+<span class="codefrag">'bin/hadoop dfsadmin'</span>
|
|
|
+ command supports a few HDFS administration related operations.
|
|
|
+ <span class="codefrag">bin/hadoop dfsadmin -help</span>
|
|
|
+ lists all the commands currently supported. For e.g.:
|
|
|
+ </p>
|
|
|
+<ul>
|
|
|
+
|
|
|
+<li>
|
|
|
+
|
|
|
+<span class="codefrag">-report</span>
|
|
|
+ : reports basic stats of HDFS. Some of this information is
|
|
|
+ also available on the Namenode front page.
|
|
|
+ </li>
|
|
|
+
|
|
|
+<li>
|
|
|
+
|
|
|
+<span class="codefrag">-safemode</span>
|
|
|
+ : though usually not required, an administrator can manually enter
|
|
|
+ or leave <em>safemode</em>.
|
|
|
+ </li>
|
|
|
+
|
|
|
+<li>
|
|
|
+
|
|
|
+<span class="codefrag">-finalizeUpgrade</span>
|
|
|
+ : removes previous backup of the cluster made during last upgrade.
|
|
|
+ </li>
|
|
|
+
|
|
|
+</ul>
|
|
|
+</div>
|
|
|
+<a name="N100E5"></a><a name="Secondary+Namenode"></a>
|
|
|
+<h2 class="h3"> Secondary Namenode </h2>
|
|
|
+<div class="section">
|
|
|
+<p>
|
|
|
+ Namenode stores modifications to the filesystem as a log
|
|
|
+ appended to a native filesystem file (<span class="codefrag">edits</span>).
|
|
|
+ When a Namenode starts up, it reads HDFS state from an image
|
|
|
+ file (<span class="codefrag">fsimage</span>) and then applies <em>edits</em> from
|
|
|
+ edits log file. It then writes new HDFS state to (<span class="codefrag">fsimage</span>)
|
|
|
+ and starts normal
|
|
|
+ operation with an empty edits file. Since namenode merges
|
|
|
+ <span class="codefrag">fsimage</span> and <span class="codefrag">edits</span> files only during start up,
|
|
|
+ edits file could get very large over time on a large cluster.
|
|
|
+ Another side effect of larger edits file is that next
|
|
|
+ restart of Namenade takes longer.
|
|
|
+ </p>
|
|
|
+<p>
|
|
|
+ The secondary namenode merges fsimage and edits log periodically
|
|
|
+ and keeps edits log size with in a limit. It is usually run on a
|
|
|
+ different machine than the primary Namenode since its memory requirements
|
|
|
+ are on the same order as the primary namemode. The secondary
|
|
|
+ namenode is started by <span class="codefrag">bin/start-dfs.sh</span> on the nodes
|
|
|
+ specified in <span class="codefrag">conf/masters</span> file.
|
|
|
+ </p>
|
|
|
+</div>
|
|
|
+<a name="N1010A"></a><a name="Rebalancer"></a>
|
|
|
+<h2 class="h3"> Rebalancer </h2>
|
|
|
+<div class="section">
|
|
|
+<p>
|
|
|
+ HDFS data might not always be be placed uniformly across the
|
|
|
+ datanode. One common reason is addition of new datanodes to an
|
|
|
+ existing cluster. While placing new <em>blocks</em> (data for a file is
|
|
|
+ stored as a series of blocks), Namenode considers various
|
|
|
+ parameters before choosing the datanodes to receive these blocks.
|
|
|
+ Some of the considerations are :
|
|
|
+ </p>
|
|
|
+<ul>
|
|
|
+
|
|
|
+<li>
|
|
|
+ Policy to keep one of the replicas of a block on the same node
|
|
|
+ as the node that is writing the block.
|
|
|
+ </li>
|
|
|
+
|
|
|
+<li>
|
|
|
+ Need to spread different replicas of a block across the racks so
|
|
|
+ that cluster can survive loss of whole rack.
|
|
|
+ </li>
|
|
|
+
|
|
|
+<li>
|
|
|
+ One of the replicas is usually placed on the same rack as the
|
|
|
+ node writing to the file so that cross-rack network I/O is
|
|
|
+ reduced.
|
|
|
+ </li>
|
|
|
+
|
|
|
+<li>
|
|
|
+ Spread HDFS data uniformly across the datanodes in the cluster.
|
|
|
+ </li>
|
|
|
+
|
|
|
+</ul>
|
|
|
+<p>
|
|
|
+ Due to multiple competing considerations, data might not be
|
|
|
+ uniformly placed across the datanodes.
|
|
|
+ HDFS provides a tool for administrators that analyzes block
|
|
|
+ placement and relanaces data across the datnodes. A brief
|
|
|
+ adminstrator's guide for rebalancer as a
|
|
|
+ <a href="http://issues.apache.org/jira/secure/attachment/12368261/RebalanceDesign6.pdf">PDF</a>
|
|
|
+ is attached to
|
|
|
+ <a href="http://issues.apache.org/jira/browse/HADOOP-1652">HADOOP-1652</a>.
|
|
|
+ </p>
|
|
|
+</div>
|
|
|
+<a name="N10131"></a><a name="Rack+Awareness"></a>
|
|
|
+<h2 class="h3"> Rack Awareness </h2>
|
|
|
+<div class="section">
|
|
|
+<p>
|
|
|
+ Typically large Hadoop clusters are arranged in <em>racks</em> and
|
|
|
+ network traffic between different nodes with in the same rack is
|
|
|
+ much more desirable than network traffic across the racks. In
|
|
|
+ addition Namenode tries to place replicas of block on
|
|
|
+ multiple racks for improved fault tolerance. Hadoop lets the
|
|
|
+ cluster administrators decide which <em>rack</em> a node belongs to
|
|
|
+ through configuration variable <span class="codefrag">dfs.network.script</span>. When this
|
|
|
+ script is configured, each node runs the script to determine its
|
|
|
+ <em>rackid</em>. A default installation assumes all the nodes belong to
|
|
|
+ the same rack. This feature and configuration is further described
|
|
|
+ in <a href="http://issues.apache.org/jira/secure/attachment/12345251/Rack_aware_HDFS_proposal.pdf">PDF</a>
|
|
|
+ attached to
|
|
|
+ <a href="http://issues.apache.org/jira/browse/HADOOP-692">HADOOP-692</a>.
|
|
|
+ </p>
|
|
|
+</div>
|
|
|
+<a name="N1014F"></a><a name="Safemode"></a>
|
|
|
+<h2 class="h3"> Safemode </h2>
|
|
|
+<div class="section">
|
|
|
+<p>
|
|
|
+ During start up Namenode loads the filesystem state from
|
|
|
+ <em>fsimage</em> and <em>edits</em> log file. It then waits for datanodes
|
|
|
+ to report their blocks so that it does not prematurely start
|
|
|
+ replicating the blocks though enough replicas already exist in the
|
|
|
+ cluster. During this time Namenode stays in <em>safemode</em>. A
|
|
|
+ <em>Safemode</em>
|
|
|
+ for Namenode is essentially a read-only mode for the HDFS cluster,
|
|
|
+ where it does not allow any modifications to filesystem or blocks.
|
|
|
+ Normally Namenode gets out of safemode automatically at
|
|
|
+ the beginning. If required, HDFS could be placed in safemode explicitly
|
|
|
+ using <span class="codefrag">'bin/hadoop dfsadmin -safemode'</span> command. Namenode front
|
|
|
+ page shows whether safemode is on or off. A more detailed
|
|
|
+ description and configuration is maintained as JavaDoc for
|
|
|
+ <a href="http://hadoop.apache.org/core/docs/current/api/org/apache/hadoop/dfs/NameNode.html#setSafeMode(org.apache.hadoop.dfs.FSConstants.SafeModeAction)"><span class="codefrag">setSafeMode()</span></a>.
|
|
|
+ </p>
|
|
|
+</div>
|
|
|
+<a name="N1016D"></a><a name="Fsck"></a>
|
|
|
+<h2 class="h3"> Fsck </h2>
|
|
|
+<div class="section">
|
|
|
+<p>
|
|
|
+ HDFS supports <span class="codefrag">fsck</span> command to check for various
|
|
|
+ inconsistencies.
|
|
|
+ It it is designed for reporting problems with various
|
|
|
+ files, for e.g. missing blocks for a file or under replicated
|
|
|
+ blocks. Unlike a traditional fsck utility for native filesystems,
|
|
|
+ this command does not correct the errors it detects. Normally Namenode
|
|
|
+ automatically corrects most of the recoverable failures.
|
|
|
+ HDFS' fsck is not a
|
|
|
+ Hadoop shell command. It can be run as '<span class="codefrag">bin/hadoop fsck</span>'.
|
|
|
+ Fsck can be run on the whole filesystem or on a subset of files.
|
|
|
+ </p>
|
|
|
+</div>
|
|
|
+<a name="N1017D"></a><a name="Upgrade+and+Rollback"></a>
|
|
|
+<h2 class="h3"> Upgrade and Rollback </h2>
|
|
|
+<div class="section">
|
|
|
+<p>
|
|
|
+ When Hadoop is upgraded on an existing cluster, as with any
|
|
|
+ software upgrade, it is possible there are new bugs or
|
|
|
+ incompatible changes that affect existing applications and were
|
|
|
+ not discovered earlier. In any non-trivial HDFS installation, it
|
|
|
+ is not an option to loose any data, let alone to restart HDFS from
|
|
|
+ scratch. HDFS allows administrators to go back to earlier version
|
|
|
+ of Hadoop and <em>roll back</em> the cluster to the state it was in
|
|
|
+ before
|
|
|
+ the upgrade. HDFS upgrade is described in more detail in
|
|
|
+ <a href="http://wiki.apache.org/hadoop/Hadoop%20Upgrade">upgrade wiki</a>.
|
|
|
+ HDFS can have one such backup at a time. Before upgrading,
|
|
|
+ administrators need to remove existing backup using <span class="codefrag">bin/hadoop
|
|
|
+ dfsadmin -finalizeUpgrade</span> command. The following
|
|
|
+ briefly describes typical upgrade procedure :
|
|
|
+ </p>
|
|
|
+<ul>
|
|
|
+
|
|
|
+<li>
|
|
|
+ Before upgrading Hadoop software,
|
|
|
+ <em>finalize</em> if there an existing backup.
|
|
|
+ <span class="codefrag">dfsadmin -upgradeProgress status</span>
|
|
|
+ can tell if the cluster needs to be <em>finalized</em>.
|
|
|
+ </li>
|
|
|
+
|
|
|
+<li>Stop the cluster and distribute new version of Hadoop.</li>
|
|
|
+
|
|
|
+<li>
|
|
|
+ Run the new version with <span class="codefrag">-upgrade</span> option
|
|
|
+ (<span class="codefrag">bin/start-dfs.sh -upgrade</span>).
|
|
|
+ </li>
|
|
|
+
|
|
|
+<li>
|
|
|
+ Most of the time, cluster works just fine. Once the new HDFS is
|
|
|
+ considered working well (may be after a few days of operation),
|
|
|
+ finalize the upgrade. Note that until the cluster is finalized,
|
|
|
+ deleting the files that existed before the upgrade does not free
|
|
|
+ up real disk space on the datanodes.
|
|
|
+ </li>
|
|
|
+
|
|
|
+<li>
|
|
|
+ If there is a need to move back to the old version,
|
|
|
+ <ul>
|
|
|
+
|
|
|
+<li> stop the cluster and distribute earlier version of Hadoop. </li>
|
|
|
+
|
|
|
+<li> start the cluster with rollback option.
|
|
|
+ (<span class="codefrag">bin/start-dfs.h -rollback</span>).
|
|
|
+ </li>
|
|
|
+
|
|
|
+</ul>
|
|
|
+
|
|
|
+</li>
|
|
|
+
|
|
|
+</ul>
|
|
|
+</div>
|
|
|
+<a name="N101BE"></a><a name="File+Permissions+and+Security"></a>
|
|
|
+<h2 class="h3"> File Permissions and Security </h2>
|
|
|
+<div class="section">
|
|
|
+<p>
|
|
|
+ The file permissions are designed to be similar to file permissions on
|
|
|
+ other familiar platforms like Linux. Currently, security is limited
|
|
|
+ to simple file permissions. The user that starts Namenode is
|
|
|
+ treated as the <em>super user</em> for HDFS. Future versions of HDFS will
|
|
|
+ support network authentication protocols like Kerberos for user
|
|
|
+ authentication and encryption of data transfers.
|
|
|
+ </p>
|
|
|
+</div>
|
|
|
+<a name="N101CB"></a><a name="Scalability"></a>
|
|
|
+<h2 class="h3"> Scalability </h2>
|
|
|
+<div class="section">
|
|
|
+<p>
|
|
|
+ Hadoop currently runs on clusters with thousands of nodes.
|
|
|
+ <a href="http://wiki.apache.org/hadoop/PoweredBy">PoweredBy Hadoop</a>
|
|
|
+ lists some of the organizations that deploy Hadoop on large
|
|
|
+ clusters. HDFS has one Namenode for each cluster. Currently
|
|
|
+ the total memory available on Namenode is the primary scalability
|
|
|
+ limitation. On very large clusters, increasing average size of
|
|
|
+ files stored in HDFS helps with increasing cluster size without
|
|
|
+ increasing memory requirements on Namenode.
|
|
|
+
|
|
|
+ The default configuration may not suite very large clustes.
|
|
|
+ <a href="http://wiki.apache.org/hadoop/FAQ">Hadoop FAQ</a> page lists
|
|
|
+ suggested configuration improvements for large Hadoop clusters.
|
|
|
+ </p>
|
|
|
+</div>
|
|
|
+<a name="N101DD"></a><a name="Related+Documentation"></a>
|
|
|
+<h2 class="h3"> Related Documentation </h2>
|
|
|
+<div class="section">
|
|
|
+<p>
|
|
|
+ This user guide is intended to be a good starting point for
|
|
|
+ working with HDFS. While it continues to improve,
|
|
|
+ there is a large wealth of documentation about Hadoop and HDFS.
|
|
|
+ The following lists starting points for further exploration :
|
|
|
+ </p>
|
|
|
+<ul>
|
|
|
+
|
|
|
+<li>
|
|
|
+
|
|
|
+<a href="http://hadoop.apache.org/">Hadoop Home Page</a>
|
|
|
+ : the start page for everything Hadoop.
|
|
|
+ </li>
|
|
|
+
|
|
|
+<li>
|
|
|
+
|
|
|
+<a href="http://wiki.apache.org/hadoop/FrontPage">Hadoop Wiki</a>
|
|
|
+ : Front page for Hadoop Wiki documentation. Unlike this
|
|
|
+ guide which is part of Hadoop source tree, Hadoop Wiki is
|
|
|
+ regularly edited by Hadoop Community.
|
|
|
+ </li>
|
|
|
+
|
|
|
+<li>
|
|
|
+<a href="http://wiki.apache.org/hadoop/FAQ">FAQ</a> from Hadoop Wiki.
|
|
|
+ </li>
|
|
|
+
|
|
|
+<li>
|
|
|
+ Hadoop <a href="http://hadoop.apache.org/core/docs/current/api/">
|
|
|
+ JavaDoc API</a>.
|
|
|
+ </li>
|
|
|
+
|
|
|
+<li>
|
|
|
+ Hadoop User Mailing List :
|
|
|
+ <a href="mailto:core-user@hadoop.apache.org">core-user[at]hadoop.apache.org</a>.
|
|
|
+ </li>
|
|
|
+
|
|
|
+<li>
|
|
|
+ Explore <span class="codefrag">conf/hadoop-default.xml</span>.
|
|
|
+ It includes brief
|
|
|
+ description of most of the configuration variables available.
|
|
|
+ </li>
|
|
|
+
|
|
|
+</ul>
|
|
|
+</div>
|
|
|
+
|
|
|
+
|
|
|
+</div>
|
|
|
+<!--+
|
|
|
+ |end content
|
|
|
+ +-->
|
|
|
+<div class="clearboth"> </div>
|
|
|
+</div>
|
|
|
+<div id="footer">
|
|
|
+<!--+
|
|
|
+ |start bottomstrip
|
|
|
+ +-->
|
|
|
+<div class="lastmodified">
|
|
|
+<script type="text/javascript"><!--
|
|
|
+document.write("Last Published: " + document.lastModified);
|
|
|
+// --></script>
|
|
|
+</div>
|
|
|
+<div class="copyright">
|
|
|
+ Copyright ©
|
|
|
+ 2007 <a href="http://www.apache.org/licenses/">The Apache Software Foundation.</a>
|
|
|
+</div>
|
|
|
+<!--+
|
|
|
+ |end bottomstrip
|
|
|
+ +-->
|
|
|
+</div>
|
|
|
+</body>
|
|
|
+</html>
|
Nigel Daley