| <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> |
| <html> |
| <head> |
| <META http-equiv="Content-Type" content="text/html; charset=UTF-8"> |
| <meta content="Apache Forrest" name="Generator"> |
| <meta name="Forrest-version" content="0.8"> |
| <meta name="Forrest-skin-name" content="pelt"> |
| <title>DistCp</title> |
| <link type="text/css" href="skin/basic.css" rel="stylesheet"> |
| <link media="screen" type="text/css" href="skin/screen.css" rel="stylesheet"> |
| <link media="print" type="text/css" href="skin/print.css" rel="stylesheet"> |
| <link type="text/css" href="skin/profile.css" rel="stylesheet"> |
| <script src="skin/getBlank.js" language="javascript" type="text/javascript"></script><script src="skin/getMenu.js" language="javascript" type="text/javascript"></script><script src="skin/fontsize.js" language="javascript" type="text/javascript"></script> |
| <link rel="shortcut icon" href="images/favicon.ico"> |
| </head> |
| <body onload="init()"> |
| <script type="text/javascript">ndeSetTextSize();</script> |
| <div id="top"> |
| <!--+ |
| |breadtrail |
| +--> |
| <div class="breadtrail"> |
| <a href="http://www.apache.org/">Apache</a> > <a href="http://hadoop.apache.org/">Hadoop</a> > <a href="http://hadoop.apache.org/core/">Core</a><script src="skin/breadcrumbs.js" language="JavaScript" type="text/javascript"></script> |
| </div> |
| <!--+ |
| |header |
| +--> |
| <div class="header"> |
| <!--+ |
| |start group logo |
| +--> |
| <div class="grouplogo"> |
| <a href="http://hadoop.apache.org/"><img class="logoImage" alt="Hadoop" src="images/hadoop-logo.jpg" title="Apache Hadoop"></a> |
| </div> |
| <!--+ |
| |end group logo |
| +--> |
| <!--+ |
| |start Project Logo |
| +--> |
| <div class="projectlogo"> |
| <a href="http://hadoop.apache.org/core/"><img class="logoImage" alt="Hadoop" src="images/core-logo.gif" title="Scalable Computing Platform"></a> |
| </div> |
| <!--+ |
| |end Project Logo |
| +--> |
| <!--+ |
| |start Search |
| +--> |
| <div class="searchbox"> |
| <form action="http://www.google.com/search" method="get" class="roundtopsmall"> |
| <input value="hadoop.apache.org" name="sitesearch" type="hidden"><input onFocus="getBlank (this, 'Search the site with google');" size="25" name="q" id="query" type="text" value="Search the site with google"> |
| <input name="Search" value="Search" type="submit"> |
| </form> |
| </div> |
| <!--+ |
| |end search |
| +--> |
| <!--+ |
| |start Tabs |
| +--> |
| <ul id="tabs"> |
| <li> |
| <a class="unselected" href="http://hadoop.apache.org/core/">Project</a> |
| </li> |
| <li> |
| <a class="unselected" href="http://wiki.apache.org/hadoop">Wiki</a> |
| </li> |
| <li class="current"> |
| <a class="selected" href="index.html">Hadoop 0.18 Documentation</a> |
| </li> |
| </ul> |
| <!--+ |
| |end Tabs |
| +--> |
| </div> |
| </div> |
| <div id="main"> |
| <div id="publishedStrip"> |
| <!--+ |
| |start Subtabs |
| +--> |
| <div id="level2tabs"></div> |
| <!--+ |
| |end Endtabs |
| +--> |
| <script type="text/javascript"><!-- |
| document.write("Last Published: " + document.lastModified); |
| // --></script> |
| </div> |
| <!--+ |
| |breadtrail |
| +--> |
| <div class="breadtrail"> |
| |
| |
| </div> |
| <!--+ |
| |start Menu, mainarea |
| +--> |
| <!--+ |
| |start Menu |
| +--> |
| <div id="menu"> |
| <div onclick="SwitchMenu('menu_selected_1.1', 'skin/')" id="menu_selected_1.1Title" class="menutitle" style="background-image: url('skin/images/chapter_open.gif');">Documentation</div> |
| <div id="menu_selected_1.1" class="selectedmenuitemgroup" style="display: block;"> |
| <div class="menuitem"> |
| <a href="index.html">Overview</a> |
| </div> |
| <div class="menuitem"> |
| <a href="quickstart.html">Quickstart</a> |
| </div> |
| <div class="menuitem"> |
| <a href="cluster_setup.html">Cluster Setup</a> |
| </div> |
| <div class="menuitem"> |
| <a href="hdfs_design.html">HDFS Architecture</a> |
| </div> |
| <div class="menuitem"> |
| <a href="hdfs_user_guide.html">HDFS User Guide</a> |
| </div> |
| <div class="menuitem"> |
| <a href="hdfs_permissions_guide.html">HDFS Permissions Guide</a> |
| </div> |
| <div class="menuitem"> |
| <a href="hdfs_quota_admin_guide.html">HDFS Quotas Administrator Guide</a> |
| </div> |
| <div class="menuitem"> |
| <a href="commands_manual.html">Commands Manual</a> |
| </div> |
| <div class="menuitem"> |
| <a href="hdfs_shell.html">FS Shell Guide</a> |
| </div> |
| <div class="menupage"> |
| <div class="menupagetitle">DistCp Guide</div> |
| </div> |
| <div class="menuitem"> |
| <a href="mapred_tutorial.html">Map-Reduce Tutorial</a> |
| </div> |
| <div class="menuitem"> |
| <a href="native_libraries.html">Native Hadoop Libraries</a> |
| </div> |
| <div class="menuitem"> |
| <a href="streaming.html">Streaming</a> |
| </div> |
| <div class="menuitem"> |
| <a href="hadoop_archives.html">Hadoop Archives</a> |
| </div> |
| <div class="menuitem"> |
| <a href="hod.html">Hadoop On Demand</a> |
| </div> |
| <div class="menuitem"> |
| <a href="api/index.html">API Docs</a> |
| </div> |
| <div class="menuitem"> |
| <a href="http://wiki.apache.org/hadoop/">Wiki</a> |
| </div> |
| <div class="menuitem"> |
| <a href="http://wiki.apache.org/hadoop/FAQ">FAQ</a> |
| </div> |
| <div class="menuitem"> |
| <a href="http://hadoop.apache.org/core/mailing_lists.html">Mailing Lists</a> |
| </div> |
| <div class="menuitem"> |
| <a href="releasenotes.html">Release Notes</a> |
| </div> |
| <div class="menuitem"> |
| <a href="changes.html">All Changes</a> |
| </div> |
| </div> |
| <div id="credit"></div> |
| <div id="roundbottom"> |
| <img style="display: none" class="corner" height="15" width="15" alt="" src="skin/images/rc-b-l-15-1body-2menu-3menu.png"></div> |
| <!--+ |
| |alternative credits |
| +--> |
| <div id="credit2"></div> |
| </div> |
| <!--+ |
| |end Menu |
| +--> |
| <!--+ |
| |start content |
| +--> |
| <div id="content"> |
| <div title="Portable Document Format" class="pdflink"> |
| <a class="dida" href="distcp.pdf"><img alt="PDF -icon" src="skin/images/pdfdoc.gif" class="skin"><br> |
| PDF</a> |
| </div> |
| <h1>DistCp</h1> |
| <div id="minitoc-area"> |
| <ul class="minitoc"> |
| <li> |
| <a href="#Overview">Overview</a> |
| </li> |
| <li> |
| <a href="#Usage">Usage</a> |
| <ul class="minitoc"> |
| <li> |
| <a href="#Basic">Basic</a> |
| </li> |
| <li> |
| <a href="#options">Options</a> |
| <ul class="minitoc"> |
| <li> |
| <a href="#Option+Index">Option Index</a> |
| </li> |
| <li> |
| <a href="#uo">Update and Overwrite</a> |
| </li> |
| </ul> |
| </li> |
| </ul> |
| </li> |
| <li> |
| <a href="#etc">Appendix</a> |
| <ul class="minitoc"> |
| <li> |
| <a href="#Map+sizing">Map sizing</a> |
| </li> |
| <li> |
| <a href="#cpver">Copying between versions of HDFS</a> |
| </li> |
| <li> |
| <a href="#Map%2FReduce+and+other+side-effects">Map/Reduce and other side-effects</a> |
| </li> |
| </ul> |
| </li> |
| </ul> |
| </div> |
| |
| |
| <a name="N1000D"></a><a name="Overview"></a> |
| <h2 class="h3">Overview</h2> |
| <div class="section"> |
| <p>DistCp (distributed copy) is a tool used for large inter/intra-cluster |
| copying. It uses Map/Reduce to effect its distribution, error |
| handling and recovery, and reporting. It expands a list of files and |
| directories into input to map tasks, each of which will copy a partition |
| of the files specified in the source list. Its Map/Reduce pedigree has |
| endowed it with some quirks in both its semantics and execution. The |
| purpose of this document is to offer guidance for common tasks and to |
| elucidate its model.</p> |
| </div> |
| |
| |
| <a name="N10017"></a><a name="Usage"></a> |
| <h2 class="h3">Usage</h2> |
| <div class="section"> |
| <a name="N1001D"></a><a name="Basic"></a> |
| <h3 class="h4">Basic</h3> |
| <p>The most common invocation of DistCp is an inter-cluster copy:</p> |
| <p> |
| <span class="codefrag">bash$ hadoop distcp hdfs://nn1:8020/foo/bar \</span> |
| <br> |
| |
| <span class="codefrag"> |
| |
| hdfs://nn2:8020/bar/foo</span> |
| </p> |
| <p>This will expand the namespace under <span class="codefrag">/foo/bar</span> 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. Note |
| that DistCp expects absolute paths.</p> |
| <p>One can also specify multiple source directories on the command |
| line:</p> |
| <p> |
| <span class="codefrag">bash$ hadoop distcp hdfs://nn1:8020/foo/a \</span> |
| <br> |
| |
| <span class="codefrag"> |
| |
| hdfs://nn1:8020/foo/b \</span> |
| <br> |
| |
| <span class="codefrag"> |
| |
| hdfs://nn2:8020/bar/foo</span> |
| </p> |
| <p>Or, equivalently, from a file using the <span class="codefrag">-f</span> option:<br> |
| |
| <span class="codefrag">bash$ hadoop distcp -f hdfs://nn1:8020/srclist \</span> |
| <br> |
| |
| <span class="codefrag"> |
| |
| hdfs://nn2:8020/bar/foo</span> |
| <br> |
| </p> |
| <p>Where <span class="codefrag">srclist</span> contains<br> |
| |
| <span class="codefrag"> hdfs://nn1:8020/foo/a</span> |
| <br> |
| |
| <span class="codefrag"> hdfs://nn1:8020/foo/b</span> |
| </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 (see <a href="#etc">Appendix</a>).</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 |
| <span class="codefrag">-update</span> 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> |
| <a name="N1007E"></a><a name="options"></a> |
| <h3 class="h4">Options</h3> |
| <a name="N10084"></a><a name="Option+Index"></a> |
| <h4>Option Index</h4> |
| <table class="ForrestTable" cellspacing="1" cellpadding="4"> |
| |
| <tr> |
| <th colspan="1" rowspan="1"> Flag </th><th colspan="1" rowspan="1"> Description </th><th colspan="1" rowspan="1"> Notes </th> |
| </tr> |
| |
| |
| <tr> |
| <td colspan="1" rowspan="1"><span class="codefrag">-p[rbugp]</span></td> |
| <td colspan="1" rowspan="1">Preserve<br> |
| r: replication number<br> |
| b: block size<br> |
| u: user<br> |
| g: group<br> |
| p: permission<br> |
| </td> |
| <td colspan="1" rowspan="1">Modification times are not preserved. Also, when |
| <span class="codefrag">-update</span> is specified, status updates will |
| <strong>not</strong> be synchronized unless the file sizes |
| also differ (i.e. unless the file is re-created). |
| </td> |
| </tr> |
| |
| <tr> |
| <td colspan="1" rowspan="1"><span class="codefrag">-i</span></td> |
| <td colspan="1" rowspan="1">Ignore failures</td> |
| <td colspan="1" rowspan="1">As explained in the <a href="#etc">Appendix</a>, this option |
| will keep more accurate statistics about the copy than the |
| default case. It also preserves logs from failed copies, which |
| can be valuable for debugging. Finally, a failing map will not |
| cause the job to fail before all splits are attempted. |
| </td> |
| </tr> |
| |
| <tr> |
| <td colspan="1" rowspan="1"><span class="codefrag">-log <logdir></span></td> |
| <td colspan="1" rowspan="1">Write logs to <logdir></td> |
| <td colspan="1" rowspan="1">DistCp keeps logs of each file it attempts to copy as map |
| output. If a map fails, the log output will not be retained if |
| it is re-executed. |
| </td> |
| </tr> |
| |
| <tr> |
| <td colspan="1" rowspan="1"><span class="codefrag">-m <num_maps></span></td> |
| <td colspan="1" rowspan="1">Maximum number of simultaneous copies</td> |
| <td colspan="1" rowspan="1">Specify the number of maps to copy data. Note that more maps |
| may not necessarily improve throughput. |
| </td> |
| </tr> |
| |
| <tr> |
| <td colspan="1" rowspan="1"><span class="codefrag">-overwrite</span></td> |
| <td colspan="1" rowspan="1">Overwrite destination</td> |
| <td colspan="1" rowspan="1">If a map fails and <span class="codefrag">-i</span> is not specified, all the |
| files in the split, not only those that failed, will be recopied. |
| As discussed in the <a href="#uo">following</a>, it also changes |
| the semantics for generating destination paths, so users should |
| use this carefully. |
| </td> |
| </tr> |
| |
| <tr> |
| <td colspan="1" rowspan="1"><span class="codefrag">-update</span></td> |
| <td colspan="1" rowspan="1">Overwrite if src size different from dst size</td> |
| <td colspan="1" rowspan="1">As noted in the preceding, this is not a "sync" |
| operation. The only criterion examined is the source and |
| destination file sizes; if they differ, the source file |
| replaces the destination file. As discussed in the |
| <a href="#uo">following</a>, it also changes the semantics for |
| generating destination paths, so users should use this carefully. |
| </td> |
| </tr> |
| |
| <tr> |
| <td colspan="1" rowspan="1"><span class="codefrag">-f <urilist_uri></span></td> |
| <td colspan="1" rowspan="1">Use list at <urilist_uri> as src list</td> |
| <td colspan="1" rowspan="1">This is equivalent to listing each source on the command |
| line. The <span class="codefrag">urilist_uri</span> list should be a fully |
| qualified URI. |
| </td> |
| </tr> |
| |
| |
| </table> |
| <a name="N10136"></a><a name="uo"></a> |
| <h4>Update and Overwrite</h4> |
| <p>It's worth giving some examples of <span class="codefrag">-update</span> and |
| <span class="codefrag">-overwrite</span>. Consider a copy from <span class="codefrag">/foo/a</span> and |
| <span class="codefrag">/foo/b</span> to <span class="codefrag">/bar/foo</span>, where the sources contain |
| the following:</p> |
| <p> |
| <span class="codefrag"> hdfs://nn1:8020/foo/a</span> |
| <br> |
| |
| <span class="codefrag"> hdfs://nn1:8020/foo/a/aa</span> |
| <br> |
| |
| <span class="codefrag"> hdfs://nn1:8020/foo/a/ab</span> |
| <br> |
| |
| <span class="codefrag"> hdfs://nn1:8020/foo/b</span> |
| <br> |
| |
| <span class="codefrag"> hdfs://nn1:8020/foo/b/ba</span> |
| <br> |
| |
| <span class="codefrag"> hdfs://nn1:8020/foo/b/ab</span> |
| </p> |
| <p>If either <span class="codefrag">-update</span> or <span class="codefrag">-overwrite</span> is set, |
| then both sources will map an entry to <span class="codefrag">/bar/foo/ab</span> at the |
| destination. For both options, the contents of each source directory |
| are compared with the <strong>contents</strong> of the destination |
| directory. Rather than permit this conflict, DistCp will abort.</p> |
| <p>In the default case, both <span class="codefrag">/bar/foo/a</span> and |
| <span class="codefrag">/bar/foo/b</span> will be created and neither will collide.</p> |
| <p>Now consider a legal copy using <span class="codefrag">-update</span>:<br> |
| |
| <span class="codefrag">distcp -update hdfs://nn1:8020/foo/a \</span> |
| <br> |
| |
| <span class="codefrag"> |
| |
| hdfs://nn1:8020/foo/b \</span> |
| <br> |
| |
| <span class="codefrag"> |
| |
| hdfs://nn2:8020/bar</span> |
| </p> |
| <p>With sources/sizes:</p> |
| <p> |
| <span class="codefrag"> hdfs://nn1:8020/foo/a</span> |
| <br> |
| |
| <span class="codefrag"> hdfs://nn1:8020/foo/a/aa 32</span> |
| <br> |
| |
| <span class="codefrag"> hdfs://nn1:8020/foo/a/ab 32</span> |
| <br> |
| |
| <span class="codefrag"> hdfs://nn1:8020/foo/b</span> |
| <br> |
| |
| <span class="codefrag"> hdfs://nn1:8020/foo/b/ba 64</span> |
| <br> |
| |
| <span class="codefrag"> hdfs://nn1:8020/foo/b/bb 32</span> |
| </p> |
| <p>And destination/sizes:</p> |
| <p> |
| <span class="codefrag"> hdfs://nn2:8020/bar</span> |
| <br> |
| |
| <span class="codefrag"> hdfs://nn2:8020/bar/aa 32</span> |
| <br> |
| |
| <span class="codefrag"> hdfs://nn2:8020/bar/ba 32</span> |
| <br> |
| |
| <span class="codefrag"> hdfs://nn2:8020/bar/bb 64</span> |
| </p> |
| <p>Will effect:</p> |
| <p> |
| <span class="codefrag"> hdfs://nn2:8020/bar</span> |
| <br> |
| |
| <span class="codefrag"> hdfs://nn2:8020/bar/aa 32</span> |
| <br> |
| |
| <span class="codefrag"> hdfs://nn2:8020/bar/ab 32</span> |
| <br> |
| |
| <span class="codefrag"> hdfs://nn2:8020/bar/ba 64</span> |
| <br> |
| |
| <span class="codefrag"> hdfs://nn2:8020/bar/bb 32</span> |
| </p> |
| <p>Only <span class="codefrag">aa</span> is not overwritten on nn2. If |
| <span class="codefrag">-overwrite</span> were specified, all elements would be |
| overwritten.</p> |
| </div> <!-- Usage --> |
| |
| |
| <a name="N101E7"></a><a name="etc"></a> |
| <h2 class="h3">Appendix</h2> |
| <div class="section"> |
| <a name="N101ED"></a><a name="Map+sizing"></a> |
| <h3 class="h4">Map sizing</h3> |
| <p>DistCp makes a faint attempt to size each map comparably so that |
| each copies roughly the same number of bytes. Note that files are the |
| finest level of granularity, so increasing the number of simultaneous |
| copiers (i.e. maps) may not always increase the number of |
| simultaneous copies nor the overall throughput.</p> |
| <p>If <span class="codefrag">-m</span> is not specified, DistCp will attempt to |
| schedule work for <span class="codefrag">min (total_bytes / bytes.per.map, 20 * |
| num_task_trackers)</span> where <span class="codefrag">bytes.per.map</span> defaults |
| to 256MB.</p> |
| <p>Tuning the number of maps to the size of the source and |
| destination clusters, the size of the copy, and the available |
| bandwidth is recommended for long-running and regularly run jobs.</p> |
| <a name="N10206"></a><a name="cpver"></a> |
| <h3 class="h4">Copying between versions of HDFS</h3> |
| <p>For copying between two different versions of Hadoop, one will |
| usually use HftpFileSystem. This is a read-only FileSystem, so DistCp |
| must be run on the destination cluster (more specifically, on |
| TaskTrackers that can write to the destination cluster). Each source is |
| specified as <span class="codefrag">hftp://<dfs.http.address>/<path></span> |
| (the default <span class="codefrag">dfs.http.address</span> is |
| <namenode>:50070).</p> |
| <a name="N10216"></a><a name="Map%2FReduce+and+other+side-effects"></a> |
| <h3 class="h4">Map/Reduce and other side-effects</h3> |
| <p>As has been mentioned in the preceding, should a map fail to copy |
| one of its inputs, there will be several side-effects.</p> |
| <ul> |
| |
| |
| <li>Unless <span class="codefrag">-i</span> is specified, the logs generated by that |
| task attempt will be replaced by the previous attempt.</li> |
| |
| |
| <li>Unless <span class="codefrag">-overwrite</span> is specified, files successfully |
| copied by a previous map on a re-execution will be marked as |
| "skipped".</li> |
| |
| |
| <li>If a map fails <span class="codefrag">mapred.map.max.attempts</span> times, the |
| remaining map tasks will be killed (unless <span class="codefrag">-i</span> is |
| set).</li> |
| |
| |
| <li>If <span class="codefrag">mapred.speculative.execution</span> is set set |
| <span class="codefrag">final</span> and <span class="codefrag">true</span>, the result of the copy is |
| undefined.</li> |
| |
| |
| </ul> |
| </div> <!-- Appendix --> |
| |
| |
| </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> |
