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hadoop
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<document xmlns="http://maven.apache.org/XDOC/2.0"
          xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
          xsi:schemaLocation="http://maven.apache.org/XDOC/2.0 http://maven.apache.org/xsd/xdoc-2.0.xsd">
  <head>
    <title>Usage </title>
  </head>
  <body>
    <section name="Basic Usage">
        <p>The most common invocation of DistCp is an inter-cluster copy:</p>
        <p><code>bash$ hadoop jar hadoop-distcp.jar hdfs://nn1:8020/foo/bar \</code><br/>
           <code>                    hdfs://nn2:8020/bar/foo</code></p>

        <p>This will expand the namespace under <code>/foo/bar</code> on nn1
        into a temporary file, partition its contents among a set of map
        tasks, and start a copy on each TaskTracker from nn1 to nn2.</p>

        <p>One can also specify multiple source directories on the command
        line:</p>
        <p><code>bash$ hadoop jar hadoop-distcp.jar hdfs://nn1:8020/foo/a \</code><br/>
           <code> hdfs://nn1:8020/foo/b \</code><br/>
           <code> hdfs://nn2:8020/bar/foo</code></p>

        <p>Or, equivalently, from a file using the <code>-f</code> option:<br/>
        <code>bash$ hadoop jar hadoop-distcp.jar -f hdfs://nn1:8020/srclist \</code><br/>
        <code> hdfs://nn2:8020/bar/foo</code><br/></p>

        <p>Where <code>srclist</code> contains<br/>
        <code>hdfs://nn1:8020/foo/a</code><br/>
        <code>hdfs://nn1:8020/foo/b</code></p>

        <p>When copying from multiple sources, DistCp will abort the copy with
        an error message if two sources collide, but collisions at the
        destination are resolved per the <a href="#options">options</a>
        specified. By default, files already existing at the destination are
        skipped (i.e. not replaced by the source file). A count of skipped
        files is reported at the end of each job, but it may be inaccurate if a
        copier failed for some subset of its files, but succeeded on a later
        attempt.</p>

        <p>It is important that each TaskTracker can reach and communicate with
        both the source and destination file systems. For HDFS, both the source
        and destination must be running the same version of the protocol or use
        a backwards-compatible protocol (see <a href="#cpver">Copying Between
        Versions</a>).</p>

        <p>After a copy, it is recommended that one generates and cross-checks
        a listing of the source and destination to verify that the copy was
        truly successful. Since DistCp employs both Map/Reduce and the
        FileSystem API, issues in or between any of the three could adversely
        and silently affect the copy. Some have had success running with
        <code>-update</code> enabled to perform a second pass, but users should
        be acquainted with its semantics before attempting this.</p>

        <p>It's also worth noting that if another client is still writing to a
        source file, the copy will likely fail. Attempting to overwrite a file
        being written at the destination should also fail on HDFS. If a source
        file is (re)moved before it is copied, the copy will fail with a
        FileNotFoundException.</p>

        <p>Please refer to the detailed Command Line Reference for information
        on all the options available in DistCp.</p>
        
    </section>
    <section name="Update and Overwrite">

        <p><code>-update</code> is used to copy files from source that don't
        exist at the target, or have different contents. <code>-overwrite</code>
        overwrites target-files even if they exist at the source, or have the
        same contents.</p>

        <p><br/>Update and Overwrite options warrant special attention, since their
        handling of source-paths varies from the defaults in a very subtle manner.
        Consider a copy from <code>/source/first/</code> and
        <code>/source/second/</code> to <code>/target/</code>, where the source
        paths have the following contents:</p>

        <p><code>hdfs://nn1:8020/source/first/1</code><br/>
           <code>hdfs://nn1:8020/source/first/2</code><br/>
           <code>hdfs://nn1:8020/source/second/10</code><br/>
           <code>hdfs://nn1:8020/source/second/20</code><br/></p>

        <p><br/>When DistCp is invoked without <code>-update</code> or
        <code>-overwrite</code>, the DistCp defaults would create directories
        <code>first/</code> and <code>second/</code>, under <code>/target</code>.
        Thus:<br/></p>

        <p><code>distcp hdfs://nn1:8020/source/first hdfs://nn1:8020/source/second hdfs://nn2:8020/target</code></p>
        <p><br/>would yield the following contents in <code>/target</code>: </p>

        <p><code>hdfs://nn2:8020/target/first/1</code><br/>
           <code>hdfs://nn2:8020/target/first/2</code><br/>
           <code>hdfs://nn2:8020/target/second/10</code><br/>
           <code>hdfs://nn2:8020/target/second/20</code><br/></p>

        <p><br/>When either <code>-update</code> or <code>-overwrite</code> is
            specified, the <strong>contents</strong> of the source-directories
            are copied to target, and not the source directories themselves. Thus: </p>

        <p><code>distcp -update hdfs://nn1:8020/source/first hdfs://nn1:8020/source/second hdfs://nn2:8020/target</code></p>

        <p><br/>would yield the following contents in <code>/target</code>: </p>

        <p><code>hdfs://nn2:8020/target/1</code><br/>
           <code>hdfs://nn2:8020/target/2</code><br/>
           <code>hdfs://nn2:8020/target/10</code><br/>
           <code>hdfs://nn2:8020/target/20</code><br/></p>

        <p><br/>By extension, if both source folders contained a file with the same
        name (say, <code>0</code>), then both sources would map an entry to
        <code>/target/0</code> at the destination. Rather than to permit this
        conflict, DistCp will abort.</p>

        <p><br/>Now, consider the following copy operation:</p>

        <p><code>distcp hdfs://nn1:8020/source/first hdfs://nn1:8020/source/second hdfs://nn2:8020/target</code></p>

        <p><br/>With sources/sizes:</p>

        <p><code>hdfs://nn1:8020/source/first/1     32</code><br/>
           <code>hdfs://nn1:8020/source/first/2     32</code><br/>
           <code>hdfs://nn1:8020/source/second/10   64</code><br/>
           <code>hdfs://nn1:8020/source/second/20   32</code><br/></p>

        <p><br/>And destination/sizes:</p>

        <p><code>hdfs://nn2:8020/target/1   32</code><br/>
           <code>hdfs://nn2:8020/target/10  32</code><br/>
           <code>hdfs://nn2:8020/target/20  64</code><br/></p>

        <p><br/>Will effect: </p>

        <p><code>hdfs://nn2:8020/target/1   32</code><br/>
           <code>hdfs://nn2:8020/target/2   32</code><br/>
           <code>hdfs://nn2:8020/target/10  64</code><br/>
           <code>hdfs://nn2:8020/target/20  32</code><br/></p>

        <p><br/><code>1</code> is skipped because the file-length and contents match.
        <code>2</code> is copied because it doesn't exist at the target.
        <code>10</code> and <code>20</code> are overwritten since the contents
        don't match the source. </p>

        <p>If <code>-update</code> is used, <code>1</code> is overwritten as well.</p>

    </section>
  </body>

</document>