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<h1>Hadoop本地库</h1>
<div id="minitoc-area">
<ul class="minitoc">
<li>
<a href="#%E7%9B%AE%E7%9A%84">目的</a>
</li>
<li>
<a href="#%E7%BB%84%E4%BB%B6">组件</a>
</li>
<li>
<a href="#%E4%BD%BF%E7%94%A8%E6%96%B9%E6%B3%95">使用方法</a>
</li>
<li>
<a href="#%E6%94%AF%E6%8C%81%E7%9A%84%E5%B9%B3%E5%8F%B0">支持的平台</a>
</li>
<li>
<a href="#%E6%9E%84%E5%BB%BAHadoop%E6%9C%AC%E5%9C%B0%E5%BA%93">构建Hadoop本地库</a>
<ul class="minitoc">
<li>
<a href="#%E6%B3%A8%E6%84%8F">注意</a>
</li>
</ul>
</li>
<li>
<a href="#%E4%BD%BF%E7%94%A8DistributedCache+%E5%8A%A0%E8%BD%BD%E6%9C%AC%E5%9C%B0%E5%BA%93"> 使用DistributedCache 加载本地库</a>
</li>
</ul>
</div>
  
    
<a name="N1000D"></a><a name="%E7%9B%AE%E7%9A%84"></a>
<h2 class="h3">目的</h2>
<div class="section">
<p>
     鉴于性能问题以及某些Java类库的缺失，对于某些组件，Hadoop提供了自己的本地实现。
	这些组件保存在Hadoop的一个独立的动态链接的库里。这个库在*nix平台上叫<em>libhadoop.so</em>. 本文主要介绍本地库的使用方法以及如何构建本地库。
</p>
</div>
    
    
<a name="N1001A"></a><a name="%E7%BB%84%E4%BB%B6"></a>
<h2 class="h3">组件</h2>
<div class="section">
<p>Hadoop现在已经有以下
      <a href="http://hadoop.apache.org/core/docs/r0.18.2/api/org/apache/hadoop/io/compress/CompressionCodec.html">
      compression codecs</a>本地组件：</p>
<ul>
        
<li>
<a href="http://www.zlib.net/">zlib</a>
</li>
        
<li>
<a href="http://www.gzip.org/">gzip</a>
</li>
        
<li>
<a href="http://www.oberhumer.com/opensource/lzo/">lzo</a>
</li>
      
</ul>
<p>在以上组件中，lzo和gzip压缩编解码器必须使用hadoop本地库才能运行。
      </p>
</div>

    
<a name="N1003D"></a><a name="%E4%BD%BF%E7%94%A8%E6%96%B9%E6%B3%95"></a>
<h2 class="h3">使用方法</h2>
<div class="section">
<p>hadoop本地库的用法很简单：</p>
<ul>
        
<li>
          看一下
	<a href="#%E6%94%AF%E6%8C%81%E7%9A%84%E5%B9%B3%E5%8F%B0">支持的平台</a>.
        </li>
        
<li>
           
<a href="http://hadoop.apache.org/core/releases.html#Download">下载</a> 预构建的32位i386架构的Linux本地hadoop库（可以在hadoop发行版的<span class="codefrag">lib/native</span>目录下找到）或者自己
          <a href="#%E6%9E%84%E5%BB%BAHadoop%E6%9C%AC%E5%9C%B0%E5%BA%93">构建</a> 这些库。
        </li>
        
<li>
          确保你的平台已经安装了<strong>zlib-1.2</strong>以上版本或者<strong>lzo2.0</strong>以上版本的软件包或者两者均已安装（根据你的需要）。
        </li>
      
</ul>
<p>
<span class="codefrag">bin/hadoop</span> 脚本通过系统属性
      <em>-Djava.library.path=&lt;path&gt;</em>来确认hadoop本地库是否包含在库路径里。</p>
<p>检查hadoop日志文件可以查看hadoop库是否正常，正常情况下会看到：</p>
<p>
        
<span class="codefrag">
          DEBUG util.NativeCodeLoader - Trying to load the custom-built 
          native-hadoop library... 
        </span>
<br>
        
<span class="codefrag">
          INFO  util.NativeCodeLoader - Loaded the native-hadoop library
        </span>
      
</p>
<p>如果出错，会看到：</p>
<p>
        
<span class="codefrag">
          INFO util.NativeCodeLoader - Unable to load native-hadoop library for 
          your platform... using builtin-java classes where applicable
        </span>
      
</p>
</div>
    
    
<a name="N10086"></a><a name="%E6%94%AF%E6%8C%81%E7%9A%84%E5%B9%B3%E5%8F%B0"></a>
<h2 class="h3">支持的平台</h2>
<div class="section">
<p>Hadoop本地库只支持*nix平台，已经广泛使用在GNU/Linux平台上，但是不支持
      <a href="http://www.cygwin.com/">Cygwin</a> 
      和 <a href="http://www.apple.com/macosx">Mac OS X</a>。 
      </p>
<p>已经测试过的GNU/Linux发行版本：</p>
<ul>
        
<li>
          
<a href="http://www.redhat.com/rhel/">RHEL4</a>/<a href="http://fedora.redhat.com/">Fedora</a>
        
</li>
        
<li>
<a href="http://www.ubuntu.com/">Ubuntu</a>
</li>
        
<li>
<a href="http://www.gentoo.org/">Gentoo</a>
</li>
      
</ul>
<p>在上述平台上，32/64位Hadoop本地库分别能和32/64位的jvm一起正常运行。
      </p>
</div>
    
    
<a name="N100B6"></a><a name="%E6%9E%84%E5%BB%BAHadoop%E6%9C%AC%E5%9C%B0%E5%BA%93"></a>
<h2 class="h3">构建Hadoop本地库</h2>
<div class="section">
<p>Hadoop本地库使用
      <a href="http://en.wikipedia.org/wiki/ANSI_C">ANSI C</a> 编写，使用GNU autotools工具链 (autoconf, autoheader, automake, autoscan, libtool)构建。也就是说构建hadoop库的平台需要有标准C的编译器和GNU autotools工具链。请参看
      <a href="#%E6%94%AF%E6%8C%81%E7%9A%84%E5%B9%B3%E5%8F%B0">支持的平台</a>。</p>
<p>你的目标平台上可能会需要的软件包：
      </p>
<ul>
        
<li>
          C 编译器 (e.g. <a href="http://gcc.gnu.org/">GNU C Compiler</a>)
        </li>
        
<li>
          GNU Autools 工具链: 
          <a href="http://www.gnu.org/software/autoconf/">autoconf</a>, 
          <a href="http://www.gnu.org/software/automake/">automake</a>, 
          <a href="http://www.gnu.org/software/libtool/">libtool</a>
        
</li>
        
<li> 
          zlib开发包 (stable version &gt;= 1.2.0)
        </li>
        
<li> 
          lzo开发包 (stable version &gt;= 2.0)
        </li> 
      
</ul>
<p>如果已经满足了上述先决条件，可以使用<span class="codefrag">build.xml</span> 
      文件，并把其中的<span class="codefrag">compile.native</span>置为 
      <span class="codefrag">true</span>，这样就可以生成hadoop本地库：</p>
<p>
<span class="codefrag">$ ant -Dcompile.native=true &lt;target&gt;</span>
</p>
<p>因为不是所有用户都需要Hadoop本地库，所以默认情况下hadoop不生成该库。
      </p>
<p>你可以在下面的路径查看新生成的hadoop本地库：</p>
<p>
<span class="codefrag">$ build/native/&lt;platform&gt;/lib</span>
</p>
<p>其中&lt;platform&gt;是下列系统属性的组合 
      <span class="codefrag">${os.name}-${os.arch}-${sun.arch.data.model}</span>；例如 
      Linux-i386-32。</p>
<a name="N10109"></a><a name="%E6%B3%A8%E6%84%8F"></a>
<h3 class="h4">注意</h3>
<ul>
          
<li>
            在生成hadoop本地库的目标平台上<strong>必须</strong> 安装了zlib和lzo开发包；但是如果你只希望使用其中一个的话，在部署时，安装其中任何一个都是足够的。
          </li>
          
<li>
		  在目标平台上生成以及部署hadoop本地库时，都需要根据32/64位jvm选取对应的32/64位zlib/lzo软件包。
          </li>
        
</ul>
</div>
<!--DCCOMMENT:diff begin-->
    
<a name="N1011F"></a><a name="%E4%BD%BF%E7%94%A8DistributedCache+%E5%8A%A0%E8%BD%BD%E6%9C%AC%E5%9C%B0%E5%BA%93"></a>
<h2 class="h3"> 使用DistributedCache 加载本地库</h2>
<div class="section">
<p>用户可以通过
      <a href="mapred_tutorial.html#DistributedCache">DistributedCache</a>
      加载本地共享库，并<em>分发</em>和建立库文件的<em>符号链接</em>。
      </p>
<p>这个例子描述了如何分发库文件并在从map/reduce任务中装载库文件。
      </p>
<ol>
      
<li>首先拷贝库文件到HDFS。<br>
      
<span class="codefrag">bin/hadoop fs -copyFromLocal mylib.so.1 /libraries/mylib.so.1</span>
      
</li>
      
<li>启动作业时包含以下代码：<br>
      
<span class="codefrag"> DistributedCache.createSymlink(conf); </span> 
<br>
      
<span class="codefrag"> DistributedCache.addCacheFile("hdfs://host:port/libraries/mylib.so.1#mylib.so", conf);
      </span>
      
</li>
      
<li>map/reduce任务中包含以下代码：<br>
      
<span class="codefrag"> System.loadLibrary("mylib.so"); </span>
      
</li>
      
</ol>
</div>
  
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