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  186. <h1>DistCp</h1>
  187. <div id="minitoc-area">
  188. <ul class="minitoc">
  189. <li>
  190. <a href="#Overview">Overview</a>
  191. </li>
  192. <li>
  193. <a href="#Usage">Usage</a>
  194. <ul class="minitoc">
  195. <li>
  196. <a href="#Basic">Basic</a>
  197. </li>
  198. <li>
  199. <a href="#options">Options</a>
  200. <ul class="minitoc">
  201. <li>
  202. <a href="#Option+Index">Option Index</a>
  203. </li>
  204. <li>
  205. <a href="#uo">Update and Overwrite</a>
  206. </li>
  207. </ul>
  208. </li>
  209. </ul>
  210. </li>
  211. <li>
  212. <a href="#etc">Appendix</a>
  213. <ul class="minitoc">
  214. <li>
  215. <a href="#Map+sizing">Map sizing</a>
  216. </li>
  217. <li>
  218. <a href="#cpver">Copying between versions of HDFS</a>
  219. </li>
  220. <li>
  221. <a href="#Map%2FReduce+and+other+side-effects">Map/Reduce and other side-effects</a>
  222. </li>
  223. </ul>
  224. </li>
  225. </ul>
  226. </div>
  227. <a name="N1000D"></a><a name="Overview"></a>
  228. <h2 class="h3">Overview</h2>
  229. <div class="section">
  230. <p>DistCp (distributed copy) is a tool used for large inter/intra-cluster
  231. copying. It uses map/reduce to effect its distribution, error
  232. handling/recovery, and reporting. It expands a list of files and
  233. directories into input to map tasks, each of which will copy a partition
  234. of the files specified in the source list. Its map/reduce pedigree has
  235. endowed it with some quirks in both its semantics and execution. The
  236. purpose of this document is to offer guidance for common tasks and to
  237. elucidate its model.</p>
  238. </div>
  239. <a name="N10017"></a><a name="Usage"></a>
  240. <h2 class="h3">Usage</h2>
  241. <div class="section">
  242. <a name="N1001D"></a><a name="Basic"></a>
  243. <h3 class="h4">Basic</h3>
  244. <p>The most common invocation of DistCp is an inter-cluster copy:</p>
  245. <p>
  246. <span class="codefrag">bash$ hadoop distcp hdfs://nn1:8020/foo/bar \</span>
  247. <br>
  248. <span class="codefrag">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
  249. &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
  250. hdfs://nn2:8020/bar/foo</span>
  251. </p>
  252. <p>This will expand the namespace under <span class="codefrag">/foo/bar</span> on nn1
  253. into a temporary file, partition its contents among a set of map
  254. tasks, and start a copy on each TaskTracker from nn1 to nn2. Note
  255. that DistCp expects absolute paths.</p>
  256. <p>One can also specify multiple source directories on the command
  257. line:</p>
  258. <p>
  259. <span class="codefrag">bash$ hadoop distcp hdfs://nn1:8020/foo/a \</span>
  260. <br>
  261. <span class="codefrag">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
  262. &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
  263. hdfs://nn1:8020/foo/b \</span>
  264. <br>
  265. <span class="codefrag">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
  266. &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
  267. hdfs://nn2:8020/bar/foo</span>
  268. </p>
  269. <p>Or, equivalently, from a file using the <span class="codefrag">-f</span> option:<br>
  270. <span class="codefrag">bash$ hadoop distcp -f hdfs://nn1:8020/srclist \</span>
  271. <br>
  272. <span class="codefrag">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
  273. &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
  274. &nbsp;hdfs://nn2:8020/bar/foo</span>
  275. <br>
  276. </p>
  277. <p>Where <span class="codefrag">srclist</span> contains<br>
  278. <span class="codefrag">&nbsp;&nbsp;&nbsp;&nbsp;hdfs://nn1:8020/foo/a</span>
  279. <br>
  280. <span class="codefrag">&nbsp;&nbsp;&nbsp;&nbsp;hdfs://nn1:8020/foo/b</span>
  281. </p>
  282. <p>When copying from multiple sources, DistCp will abort the copy with
  283. an error message if two sources collide, but collisions at the
  284. destination are resolved per the <a href="#options">options</a>
  285. specified. By default, files already existing at the destination are
  286. skipped (i.e. not replaced by the source file). A count of skipped
  287. files is reported at the end of each job, but it may be inaccurate if a
  288. copier failed for some subset of its files, but succeeded on a later
  289. attempt (see <a href="#etc">Appendix</a>).</p>
  290. <p>It is important that each TaskTracker can reach and communicate with
  291. both the source and destination filesystems. For hdfs, both the source
  292. and destination must be running the same version of the protocol or use
  293. a backwards-compatible protocol (see <a href="#cpver">Copying Between
  294. Versions</a>).</p>
  295. <p>After a copy, it is recommended that one generates and cross-checks
  296. a listing of the source and destination to verify that the copy was
  297. truly successful. Since DistCp employs both map/reduce and the
  298. FileSystem API, issues in or between any of the three could adversely
  299. and silently affect the copy. Some have had success running with
  300. <span class="codefrag">-update</span> enabled to perform a second pass, but users should
  301. be acquainted with its semantics before attempting this.</p>
  302. <p>It's also worth noting that if another client is still writing to a
  303. source file, the copy will likely fail. Attempting to overwrite a file
  304. being written at the destination should also fail on HDFS. If a source
  305. file is (re)moved before it is copied, the copy will fail with a
  306. FileNotFoundException.</p>
  307. <a name="N1007E"></a><a name="options"></a>
  308. <h3 class="h4">Options</h3>
  309. <a name="N10084"></a><a name="Option+Index"></a>
  310. <h4>Option Index</h4>
  311. <table class="ForrestTable" cellspacing="1" cellpadding="4">
  312. <tr>
  313. <th colspan="1" rowspan="1"> Flag </th><th colspan="1" rowspan="1"> Description </th><th colspan="1" rowspan="1"> Notes </th>
  314. </tr>
  315. <tr>
  316. <td colspan="1" rowspan="1"><span class="codefrag">-p[rbugp]</span></td>
  317. <td colspan="1" rowspan="1">Preserve<br>
  318. &nbsp;&nbsp;r: replication number<br>
  319. &nbsp;&nbsp;b: block size<br>
  320. &nbsp;&nbsp;u: user<br>
  321. &nbsp;&nbsp;g: group<br>
  322. &nbsp;&nbsp;p: permission<br>
  323. </td>
  324. <td colspan="1" rowspan="1">Modification times are not preserved. Also, when
  325. <span class="codefrag">-update</span> is specified, status updates will
  326. <strong>not</strong> be synchronized unless the file sizes
  327. also differ (i.e. unless the file is re-created).
  328. </td>
  329. </tr>
  330. <tr>
  331. <td colspan="1" rowspan="1"><span class="codefrag">-i</span></td>
  332. <td colspan="1" rowspan="1">Ignore failures</td>
  333. <td colspan="1" rowspan="1">As explained in the <a href="#etc">Appendix</a>, this option
  334. will keep more accurate statistics about the copy than the
  335. default case. It also preserves logs from failed copies, which
  336. can be valuable for debugging. Finally, a failing map will not
  337. cause the job to fail before all splits are attempted.
  338. </td>
  339. </tr>
  340. <tr>
  341. <td colspan="1" rowspan="1"><span class="codefrag">-log &lt;logdir&gt;</span></td>
  342. <td colspan="1" rowspan="1">Write logs to &lt;logdir&gt;</td>
  343. <td colspan="1" rowspan="1">DistCp keeps logs of each file it attempts to copy as map
  344. output. If a map fails, the log output will not be retained if
  345. it is re-executed.
  346. </td>
  347. </tr>
  348. <tr>
  349. <td colspan="1" rowspan="1"><span class="codefrag">-m &lt;num_maps&gt;</span></td>
  350. <td colspan="1" rowspan="1">Maximum number of simultaneous copies</td>
  351. <td colspan="1" rowspan="1">Specify the number of maps to copy data. Note that more maps
  352. may not necessarily improve throughput.
  353. </td>
  354. </tr>
  355. <tr>
  356. <td colspan="1" rowspan="1"><span class="codefrag">-overwrite</span></td>
  357. <td colspan="1" rowspan="1">Overwrite destination</td>
  358. <td colspan="1" rowspan="1">If a map fails and <span class="codefrag">-i</span> is not specified, all the
  359. files in the split, not only those that failed, will be recopied.
  360. As discussed in the <a href="#uo">following</a>, it also changes
  361. the semantics for generating destination paths, so users should
  362. use this carefully.
  363. </td>
  364. </tr>
  365. <tr>
  366. <td colspan="1" rowspan="1"><span class="codefrag">-update</span></td>
  367. <td colspan="1" rowspan="1">Overwrite if src size different from dst size</td>
  368. <td colspan="1" rowspan="1">As noted in the preceding, this is not a "sync"
  369. operation. The only criterion examined is the source and
  370. destination file sizes; if they differ, the source file
  371. replaces the destination file. As discussed in the
  372. <a href="#uo">following</a>, it also changes the semantics for
  373. generating destination paths, so users should use this carefully.
  374. </td>
  375. </tr>
  376. <tr>
  377. <td colspan="1" rowspan="1"><span class="codefrag">-f &lt;urilist_uri&gt;</span></td>
  378. <td colspan="1" rowspan="1">Use list at &lt;urilist_uri&gt; as src list</td>
  379. <td colspan="1" rowspan="1">This is equivalent to listing each source on the command
  380. line. The <span class="codefrag">urilist_uri</span> list should be a fully
  381. qualified URI.
  382. </td>
  383. </tr>
  384. </table>
  385. <a name="N10136"></a><a name="uo"></a>
  386. <h4>Update and Overwrite</h4>
  387. <p>It's worth giving some examples of <span class="codefrag">-update</span> and
  388. <span class="codefrag">-overwrite</span>. Consider a copy from <span class="codefrag">/foo/a</span> and
  389. <span class="codefrag">/foo/b</span> to <span class="codefrag">/bar/foo</span>, where the sources contain
  390. the following:</p>
  391. <p>
  392. <span class="codefrag">&nbsp;&nbsp;&nbsp;&nbsp;hdfs://nn1:8020/foo/a</span>
  393. <br>
  394. <span class="codefrag">&nbsp;&nbsp;&nbsp;&nbsp;hdfs://nn1:8020/foo/a/aa</span>
  395. <br>
  396. <span class="codefrag">&nbsp;&nbsp;&nbsp;&nbsp;hdfs://nn1:8020/foo/a/ab</span>
  397. <br>
  398. <span class="codefrag">&nbsp;&nbsp;&nbsp;&nbsp;hdfs://nn1:8020/foo/b</span>
  399. <br>
  400. <span class="codefrag">&nbsp;&nbsp;&nbsp;&nbsp;hdfs://nn1:8020/foo/b/ba</span>
  401. <br>
  402. <span class="codefrag">&nbsp;&nbsp;&nbsp;&nbsp;hdfs://nn1:8020/foo/b/ab</span>
  403. </p>
  404. <p>If either <span class="codefrag">-update</span> or <span class="codefrag">-overwrite</span> is set,
  405. then both sources will map an entry to <span class="codefrag">/bar/foo/ab</span> at the
  406. destination. For both options, the contents of each source directory
  407. are compared with the <strong>contents</strong> of the destination
  408. directory. Rather than permit this conflict, DistCp will abort.</p>
  409. <p>In the default case, both <span class="codefrag">/bar/foo/a</span> and
  410. <span class="codefrag">/bar/foo/b</span> will be created and neither will collide.</p>
  411. <p>Now consider a legal copy using <span class="codefrag">-update</span>:<br>
  412. <span class="codefrag">distcp -update hdfs://nn1:8020/foo/a \</span>
  413. <br>
  414. <span class="codefrag">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
  415. &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
  416. hdfs://nn1:8020/foo/b \</span>
  417. <br>
  418. <span class="codefrag">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
  419. &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
  420. hdfs://nn2:8020/bar</span>
  421. </p>
  422. <p>With sources/sizes:</p>
  423. <p>
  424. <span class="codefrag">&nbsp;&nbsp;&nbsp;&nbsp;hdfs://nn1:8020/foo/a</span>
  425. <br>
  426. <span class="codefrag">&nbsp;&nbsp;&nbsp;&nbsp;hdfs://nn1:8020/foo/a/aa 32</span>
  427. <br>
  428. <span class="codefrag">&nbsp;&nbsp;&nbsp;&nbsp;hdfs://nn1:8020/foo/a/ab 32</span>
  429. <br>
  430. <span class="codefrag">&nbsp;&nbsp;&nbsp;&nbsp;hdfs://nn1:8020/foo/b</span>
  431. <br>
  432. <span class="codefrag">&nbsp;&nbsp;&nbsp;&nbsp;hdfs://nn1:8020/foo/b/ba 64</span>
  433. <br>
  434. <span class="codefrag">&nbsp;&nbsp;&nbsp;&nbsp;hdfs://nn1:8020/foo/b/bb 32</span>
  435. </p>
  436. <p>And destination/sizes:</p>
  437. <p>
  438. <span class="codefrag">&nbsp;&nbsp;&nbsp;&nbsp;hdfs://nn2:8020/bar</span>
  439. <br>
  440. <span class="codefrag">&nbsp;&nbsp;&nbsp;&nbsp;hdfs://nn2:8020/bar/aa 32</span>
  441. <br>
  442. <span class="codefrag">&nbsp;&nbsp;&nbsp;&nbsp;hdfs://nn2:8020/bar/ba 32</span>
  443. <br>
  444. <span class="codefrag">&nbsp;&nbsp;&nbsp;&nbsp;hdfs://nn2:8020/bar/bb 64</span>
  445. </p>
  446. <p>Will effect:</p>
  447. <p>
  448. <span class="codefrag">&nbsp;&nbsp;&nbsp;&nbsp;hdfs://nn2:8020/bar</span>
  449. <br>
  450. <span class="codefrag">&nbsp;&nbsp;&nbsp;&nbsp;hdfs://nn2:8020/bar/aa 32</span>
  451. <br>
  452. <span class="codefrag">&nbsp;&nbsp;&nbsp;&nbsp;hdfs://nn2:8020/bar/ab 32</span>
  453. <br>
  454. <span class="codefrag">&nbsp;&nbsp;&nbsp;&nbsp;hdfs://nn2:8020/bar/ba 64</span>
  455. <br>
  456. <span class="codefrag">&nbsp;&nbsp;&nbsp;&nbsp;hdfs://nn2:8020/bar/bb 32</span>
  457. </p>
  458. <p>Only <span class="codefrag">aa</span> is not overwritten on nn2. If
  459. <span class="codefrag">-overwrite</span> were specified, all elements would be
  460. overwritten.</p>
  461. </div> <!-- Usage -->
  462. <a name="N101E7"></a><a name="etc"></a>
  463. <h2 class="h3">Appendix</h2>
  464. <div class="section">
  465. <a name="N101ED"></a><a name="Map+sizing"></a>
  466. <h3 class="h4">Map sizing</h3>
  467. <p>DistCp makes a faint attempt to size each map comparably so that
  468. each copies roughly the same number of bytes. Note that files are the
  469. finest level of granularity, so increasing the number of simultaneous
  470. copiers (i.e. maps) may not always increase the number of
  471. simultaneous copies nor the overall throughput.</p>
  472. <p>If <span class="codefrag">-m</span> is not specified, DistCp will attempt to
  473. schedule work for <span class="codefrag">min(total_bytes / bytes.per.map, 20 *
  474. num_task_trackers)</span> where <span class="codefrag">bytes.per.map</span> defaults
  475. to 256MB.</p>
  476. <p>Tuning the number of maps to the size of the source and
  477. destination clusters, the size of the copy, and the available
  478. bandwidth is recommended for long-running and regularly run jobs.</p>
  479. <a name="N10206"></a><a name="cpver"></a>
  480. <h3 class="h4">Copying between versions of HDFS</h3>
  481. <p>For copying between two different versions of Hadoop, one will
  482. usually use HftpFileSystem. This is a read-only FileSystem, so DistCp
  483. must be run on the destination cluster (more specifically, on
  484. TaskTrackers that can write to the destination cluster). Each source is
  485. specified as <span class="codefrag">hftp://&lt;dfs.http.address&gt;/&lt;path&gt;</span>
  486. (the default <span class="codefrag">dfs.http.address</span> is
  487. &lt;namenode&gt;:50070).</p>
  488. <a name="N10216"></a><a name="Map%2FReduce+and+other+side-effects"></a>
  489. <h3 class="h4">Map/Reduce and other side-effects</h3>
  490. <p>As has been mentioned in the preceding, should a map fail to copy
  491. one of its inputs, there will be several side-effects.</p>
  492. <ul>
  493. <li>Unless <span class="codefrag">-i</span> is specified, the logs generated by that
  494. task attempt will be replaced by the previous attempt.</li>
  495. <li>Unless <span class="codefrag">-overwrite</span> is specified, files successfully
  496. copied by a previous map on a re-execution will be marked as
  497. "skipped".</li>
  498. <li>If a map fails <span class="codefrag">mapred.map.max.attempts</span> times, the
  499. remaining map tasks will be killed (unless <span class="codefrag">-i</span> is
  500. set).</li>
  501. <li>If <span class="codefrag">mapred.speculative.execution</span> is set set
  502. <span class="codefrag">final</span> and <span class="codefrag">true</span>, the result of the copy is
  503. undefined.</li>
  504. </ul>
  505. </div> <!-- Appendix -->
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