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226 | <h1>DistCp Guide</h1> |
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227 | <div id="minitoc-area"> |
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228 | <ul class="minitoc"> |
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229 | <li> |
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230 | <a href="#Overview">Overview</a> |
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231 | </li> |
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232 | <li> |
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233 | <a href="#Usage">Usage</a> |
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234 | <ul class="minitoc"> |
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235 | <li> |
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236 | <a href="#Basic">Basic</a> |
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237 | </li> |
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238 | <li> |
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239 | <a href="#options">Options</a> |
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240 | <ul class="minitoc"> |
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241 | <li> |
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242 | <a href="#Option+Index">Option Index</a> |
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243 | </li> |
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244 | <li> |
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245 | <a href="#Symbolic-Representations">Symbolic Representations</a> |
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246 | </li> |
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247 | <li> |
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248 | <a href="#uo">Update and Overwrite</a> |
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249 | </li> |
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250 | </ul> |
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251 | </li> |
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252 | </ul> |
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253 | </li> |
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254 | <li> |
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255 | <a href="#etc">Appendix</a> |
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256 | <ul class="minitoc"> |
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257 | <li> |
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258 | <a href="#Map+sizing">Map sizing</a> |
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259 | </li> |
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260 | <li> |
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261 | <a href="#cpver">Copying between versions of HDFS</a> |
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262 | </li> |
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263 | <li> |
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264 | <a href="#Map%2FReduce+and+other+side-effects">Map/Reduce and other side-effects</a> |
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265 | </li> |
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266 | </ul> |
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267 | </li> |
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268 | </ul> |
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269 | </div> |
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270 | |
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271 | |
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272 | <a name="N1000D"></a><a name="Overview"></a> |
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273 | <h2 class="h3">Overview</h2> |
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274 | <div class="section"> |
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275 | <p>DistCp (distributed copy) is a tool used for large inter/intra-cluster |
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276 | copying. It uses Map/Reduce to effect its distribution, error |
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277 | handling and recovery, and reporting. It expands a list of files and |
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278 | directories into input to map tasks, each of which will copy a partition |
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279 | of the files specified in the source list. Its Map/Reduce pedigree has |
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280 | endowed it with some quirks in both its semantics and execution. The |
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281 | purpose of this document is to offer guidance for common tasks and to |
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282 | elucidate its model.</p> |
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283 | </div> |
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284 | |
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285 | |
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286 | <a name="N10017"></a><a name="Usage"></a> |
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287 | <h2 class="h3">Usage</h2> |
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288 | <div class="section"> |
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289 | <a name="N1001D"></a><a name="Basic"></a> |
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290 | <h3 class="h4">Basic</h3> |
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291 | <p>The most common invocation of DistCp is an inter-cluster copy:</p> |
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292 | <p> |
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293 | <span class="codefrag">bash$ hadoop distcp hdfs://nn1:8020/foo/bar \</span> |
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294 | <br> |
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295 | |
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296 | <span class="codefrag"> |
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297 | |
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298 | hdfs://nn2:8020/bar/foo</span> |
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299 | </p> |
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300 | <p>This will expand the namespace under <span class="codefrag">/foo/bar</span> on nn1 |
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301 | into a temporary file, partition its contents among a set of map |
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302 | tasks, and start a copy on each TaskTracker from nn1 to nn2. Note |
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303 | that DistCp expects absolute paths.</p> |
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304 | <p>One can also specify multiple source directories on the command |
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305 | line:</p> |
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306 | <p> |
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307 | <span class="codefrag">bash$ hadoop distcp hdfs://nn1:8020/foo/a \</span> |
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308 | <br> |
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309 | |
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310 | <span class="codefrag"> |
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311 | |
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312 | hdfs://nn1:8020/foo/b \</span> |
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313 | <br> |
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314 | |
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315 | <span class="codefrag"> |
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316 | |
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317 | hdfs://nn2:8020/bar/foo</span> |
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318 | </p> |
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319 | <p>Or, equivalently, from a file using the <span class="codefrag">-f</span> option:<br> |
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320 | |
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321 | <span class="codefrag">bash$ hadoop distcp -f hdfs://nn1:8020/srclist \</span> |
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322 | <br> |
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323 | |
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324 | <span class="codefrag"> |
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325 | |
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326 | hdfs://nn2:8020/bar/foo</span> |
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327 | <br> |
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328 | </p> |
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329 | <p>Where <span class="codefrag">srclist</span> contains<br> |
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330 | |
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331 | <span class="codefrag"> hdfs://nn1:8020/foo/a</span> |
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332 | <br> |
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333 | |
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334 | <span class="codefrag"> hdfs://nn1:8020/foo/b</span> |
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335 | </p> |
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336 | <p>When copying from multiple sources, DistCp will abort the copy with |
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337 | an error message if two sources collide, but collisions at the |
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338 | destination are resolved per the <a href="#options">options</a> |
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339 | specified. By default, files already existing at the destination are |
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340 | skipped (i.e. not replaced by the source file). A count of skipped |
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341 | files is reported at the end of each job, but it may be inaccurate if a |
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342 | copier failed for some subset of its files, but succeeded on a later |
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343 | attempt (see <a href="#etc">Appendix</a>).</p> |
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344 | <p>It is important that each TaskTracker can reach and communicate with |
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345 | both the source and destination file systems. For HDFS, both the source |
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346 | and destination must be running the same version of the protocol or use |
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347 | a backwards-compatible protocol (see <a href="#cpver">Copying Between |
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348 | Versions</a>).</p> |
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349 | <p>After a copy, it is recommended that one generates and cross-checks |
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350 | a listing of the source and destination to verify that the copy was |
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351 | truly successful. Since DistCp employs both Map/Reduce and the |
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352 | FileSystem API, issues in or between any of the three could adversely |
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353 | and silently affect the copy. Some have had success running with |
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354 | <span class="codefrag">-update</span> enabled to perform a second pass, but users should |
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355 | be acquainted with its semantics before attempting this.</p> |
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356 | <p>It's also worth noting that if another client is still writing to a |
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357 | source file, the copy will likely fail. Attempting to overwrite a file |
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358 | being written at the destination should also fail on HDFS. If a source |
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359 | file is (re)moved before it is copied, the copy will fail with a |
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360 | FileNotFoundException.</p> |
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361 | <a name="N1007E"></a><a name="options"></a> |
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362 | <h3 class="h4">Options</h3> |
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363 | <a name="N10084"></a><a name="Option+Index"></a> |
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364 | <h4>Option Index</h4> |
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365 | <table class="ForrestTable" cellspacing="1" cellpadding="4"> |
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366 | |
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367 | <tr> |
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368 | <th colspan="1" rowspan="1"> Flag </th><th colspan="1" rowspan="1"> Description </th><th colspan="1" rowspan="1"> Notes </th> |
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369 | </tr> |
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370 | |
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371 | |
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372 | <tr> |
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373 | <td colspan="1" rowspan="1"><span class="codefrag">-p[rbugp]</span></td> |
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374 | <td colspan="1" rowspan="1">Preserve<br> |
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375 | r: replication number<br> |
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376 | b: block size<br> |
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377 | u: user<br> |
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378 | g: group<br> |
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379 | p: permission<br> |
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380 | </td> |
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381 | <td colspan="1" rowspan="1">Modification times are not preserved. Also, when |
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382 | <span class="codefrag">-update</span> is specified, status updates will |
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383 | <strong>not</strong> be synchronized unless the file sizes |
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384 | also differ (i.e. unless the file is re-created). |
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385 | </td> |
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386 | </tr> |
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387 | |
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388 | <tr> |
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389 | <td colspan="1" rowspan="1"><span class="codefrag">-i</span></td> |
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390 | <td colspan="1" rowspan="1">Ignore failures</td> |
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391 | <td colspan="1" rowspan="1">As explained in the <a href="#etc">Appendix</a>, this option |
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392 | will keep more accurate statistics about the copy than the |
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393 | default case. It also preserves logs from failed copies, which |
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394 | can be valuable for debugging. Finally, a failing map will not |
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395 | cause the job to fail before all splits are attempted. |
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396 | </td> |
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397 | </tr> |
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398 | |
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399 | <tr> |
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400 | <td colspan="1" rowspan="1"><span class="codefrag">-log <logdir></span></td> |
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401 | <td colspan="1" rowspan="1">Write logs to <logdir></td> |
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402 | <td colspan="1" rowspan="1">DistCp keeps logs of each file it attempts to copy as map |
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403 | output. If a map fails, the log output will not be retained if |
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404 | it is re-executed. |
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405 | </td> |
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406 | </tr> |
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407 | |
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408 | <tr> |
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409 | <td colspan="1" rowspan="1"><span class="codefrag">-m <num_maps></span></td> |
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410 | <td colspan="1" rowspan="1">Maximum number of simultaneous copies</td> |
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411 | <td colspan="1" rowspan="1">Specify the number of maps to copy data. Note that more maps |
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412 | may not necessarily improve throughput. |
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413 | </td> |
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414 | </tr> |
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415 | |
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416 | <tr> |
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417 | <td colspan="1" rowspan="1"><span class="codefrag">-overwrite</span></td> |
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418 | <td colspan="1" rowspan="1">Overwrite destination</td> |
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419 | <td colspan="1" rowspan="1">If a map fails and <span class="codefrag">-i</span> is not specified, all the |
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420 | files in the split, not only those that failed, will be recopied. |
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421 | As discussed in the <a href="#uo">following</a>, it also changes |
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422 | the semantics for generating destination paths, so users should |
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423 | use this carefully. |
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424 | </td> |
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425 | </tr> |
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426 | |
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427 | <tr> |
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428 | <td colspan="1" rowspan="1"><span class="codefrag">-update</span></td> |
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429 | <td colspan="1" rowspan="1">Overwrite if src size different from dst size</td> |
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430 | <td colspan="1" rowspan="1">As noted in the preceding, this is not a "sync" |
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431 | operation. The only criterion examined is the source and |
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432 | destination file sizes; if they differ, the source file |
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433 | replaces the destination file. As discussed in the |
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434 | <a href="#uo">following</a>, it also changes the semantics for |
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435 | generating destination paths, so users should use this carefully. |
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436 | </td> |
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437 | </tr> |
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438 | |
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439 | <tr> |
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440 | <td colspan="1" rowspan="1"><span class="codefrag">-f <urilist_uri></span></td> |
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441 | <td colspan="1" rowspan="1">Use list at <urilist_uri> as src list</td> |
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442 | <td colspan="1" rowspan="1">This is equivalent to listing each source on the command |
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443 | line. The <span class="codefrag">urilist_uri</span> list should be a fully |
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444 | qualified URI. |
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445 | </td> |
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446 | </tr> |
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447 | |
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448 | <tr> |
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449 | <td colspan="1" rowspan="1"><span class="codefrag">-filelimit <n></span></td> |
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450 | <td colspan="1" rowspan="1">Limit the total number of files to be <= n</td> |
---|
451 | <td colspan="1" rowspan="1">See also <a href="#Symbolic-Representations">Symbolic |
---|
452 | Representations</a>. |
---|
453 | </td> |
---|
454 | </tr> |
---|
455 | |
---|
456 | <tr> |
---|
457 | <td colspan="1" rowspan="1"><span class="codefrag">-sizelimit <n></span></td> |
---|
458 | <td colspan="1" rowspan="1">Limit the total size to be <= n bytes</td> |
---|
459 | <td colspan="1" rowspan="1">See also <a href="#Symbolic-Representations">Symbolic |
---|
460 | Representations</a>. |
---|
461 | </td> |
---|
462 | </tr> |
---|
463 | |
---|
464 | <tr> |
---|
465 | <td colspan="1" rowspan="1"><span class="codefrag">-delete</span></td> |
---|
466 | <td colspan="1" rowspan="1">Delete the files existing in the dst but not in src</td> |
---|
467 | <td colspan="1" rowspan="1">The deletion is done by FS Shell. So the trash will be used, |
---|
468 | if it is enable. |
---|
469 | </td> |
---|
470 | </tr> |
---|
471 | |
---|
472 | |
---|
473 | </table> |
---|
474 | <a name="N10171"></a><a name="Symbolic-Representations"></a> |
---|
475 | <h4>Symbolic Representations</h4> |
---|
476 | <p> |
---|
477 | The parameter <n> in <span class="codefrag">-filelimit</span> |
---|
478 | and <span class="codefrag">-sizelimit</span> can be specified with symbolic |
---|
479 | representation. For examples, |
---|
480 | </p> |
---|
481 | <ul> |
---|
482 | |
---|
483 | <li>1230k = 1230 * 1024 = 1259520</li> |
---|
484 | |
---|
485 | <li>891g = 891 * 1024^3 = 956703965184</li> |
---|
486 | |
---|
487 | </ul> |
---|
488 | <a name="N1018A"></a><a name="uo"></a> |
---|
489 | <h4>Update and Overwrite</h4> |
---|
490 | <p>It's worth giving some examples of <span class="codefrag">-update</span> and |
---|
491 | <span class="codefrag">-overwrite</span>. Consider a copy from <span class="codefrag">/foo/a</span> and |
---|
492 | <span class="codefrag">/foo/b</span> to <span class="codefrag">/bar/foo</span>, where the sources contain |
---|
493 | the following:</p> |
---|
494 | <p> |
---|
495 | <span class="codefrag"> hdfs://nn1:8020/foo/a</span> |
---|
496 | <br> |
---|
497 | |
---|
498 | <span class="codefrag"> hdfs://nn1:8020/foo/a/aa</span> |
---|
499 | <br> |
---|
500 | |
---|
501 | <span class="codefrag"> hdfs://nn1:8020/foo/a/ab</span> |
---|
502 | <br> |
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503 | |
---|
504 | <span class="codefrag"> hdfs://nn1:8020/foo/b</span> |
---|
505 | <br> |
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506 | |
---|
507 | <span class="codefrag"> hdfs://nn1:8020/foo/b/ba</span> |
---|
508 | <br> |
---|
509 | |
---|
510 | <span class="codefrag"> hdfs://nn1:8020/foo/b/ab</span> |
---|
511 | </p> |
---|
512 | <p>If either <span class="codefrag">-update</span> or <span class="codefrag">-overwrite</span> is set, |
---|
513 | then both sources will map an entry to <span class="codefrag">/bar/foo/ab</span> at the |
---|
514 | destination. For both options, the contents of each source directory |
---|
515 | are compared with the <strong>contents</strong> of the destination |
---|
516 | directory. Rather than permit this conflict, DistCp will abort.</p> |
---|
517 | <p>In the default case, both <span class="codefrag">/bar/foo/a</span> and |
---|
518 | <span class="codefrag">/bar/foo/b</span> will be created and neither will collide.</p> |
---|
519 | <p>Now consider a legal copy using <span class="codefrag">-update</span>:<br> |
---|
520 | |
---|
521 | <span class="codefrag">distcp -update hdfs://nn1:8020/foo/a \</span> |
---|
522 | <br> |
---|
523 | |
---|
524 | <span class="codefrag"> |
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525 | |
---|
526 | hdfs://nn1:8020/foo/b \</span> |
---|
527 | <br> |
---|
528 | |
---|
529 | <span class="codefrag"> |
---|
530 | |
---|
531 | hdfs://nn2:8020/bar</span> |
---|
532 | </p> |
---|
533 | <p>With sources/sizes:</p> |
---|
534 | <p> |
---|
535 | <span class="codefrag"> hdfs://nn1:8020/foo/a</span> |
---|
536 | <br> |
---|
537 | |
---|
538 | <span class="codefrag"> hdfs://nn1:8020/foo/a/aa 32</span> |
---|
539 | <br> |
---|
540 | |
---|
541 | <span class="codefrag"> hdfs://nn1:8020/foo/a/ab 32</span> |
---|
542 | <br> |
---|
543 | |
---|
544 | <span class="codefrag"> hdfs://nn1:8020/foo/b</span> |
---|
545 | <br> |
---|
546 | |
---|
547 | <span class="codefrag"> hdfs://nn1:8020/foo/b/ba 64</span> |
---|
548 | <br> |
---|
549 | |
---|
550 | <span class="codefrag"> hdfs://nn1:8020/foo/b/bb 32</span> |
---|
551 | </p> |
---|
552 | <p>And destination/sizes:</p> |
---|
553 | <p> |
---|
554 | <span class="codefrag"> hdfs://nn2:8020/bar</span> |
---|
555 | <br> |
---|
556 | |
---|
557 | <span class="codefrag"> hdfs://nn2:8020/bar/aa 32</span> |
---|
558 | <br> |
---|
559 | |
---|
560 | <span class="codefrag"> hdfs://nn2:8020/bar/ba 32</span> |
---|
561 | <br> |
---|
562 | |
---|
563 | <span class="codefrag"> hdfs://nn2:8020/bar/bb 64</span> |
---|
564 | </p> |
---|
565 | <p>Will effect:</p> |
---|
566 | <p> |
---|
567 | <span class="codefrag"> hdfs://nn2:8020/bar</span> |
---|
568 | <br> |
---|
569 | |
---|
570 | <span class="codefrag"> hdfs://nn2:8020/bar/aa 32</span> |
---|
571 | <br> |
---|
572 | |
---|
573 | <span class="codefrag"> hdfs://nn2:8020/bar/ab 32</span> |
---|
574 | <br> |
---|
575 | |
---|
576 | <span class="codefrag"> hdfs://nn2:8020/bar/ba 64</span> |
---|
577 | <br> |
---|
578 | |
---|
579 | <span class="codefrag"> hdfs://nn2:8020/bar/bb 32</span> |
---|
580 | </p> |
---|
581 | <p>Only <span class="codefrag">aa</span> is not overwritten on nn2. If |
---|
582 | <span class="codefrag">-overwrite</span> were specified, all elements would be |
---|
583 | overwritten.</p> |
---|
584 | </div> <!-- Usage --> |
---|
585 | |
---|
586 | |
---|
587 | <a name="N1023B"></a><a name="etc"></a> |
---|
588 | <h2 class="h3">Appendix</h2> |
---|
589 | <div class="section"> |
---|
590 | <a name="N10241"></a><a name="Map+sizing"></a> |
---|
591 | <h3 class="h4">Map sizing</h3> |
---|
592 | <p>DistCp makes a faint attempt to size each map comparably so that |
---|
593 | each copies roughly the same number of bytes. Note that files are the |
---|
594 | finest level of granularity, so increasing the number of simultaneous |
---|
595 | copiers (i.e. maps) may not always increase the number of |
---|
596 | simultaneous copies nor the overall throughput.</p> |
---|
597 | <p>If <span class="codefrag">-m</span> is not specified, DistCp will attempt to |
---|
598 | schedule work for <span class="codefrag">min (total_bytes / bytes.per.map, 20 * |
---|
599 | num_task_trackers)</span> where <span class="codefrag">bytes.per.map</span> defaults |
---|
600 | to 256MB.</p> |
---|
601 | <p>Tuning the number of maps to the size of the source and |
---|
602 | destination clusters, the size of the copy, and the available |
---|
603 | bandwidth is recommended for long-running and regularly run jobs.</p> |
---|
604 | <a name="N1025A"></a><a name="cpver"></a> |
---|
605 | <h3 class="h4">Copying between versions of HDFS</h3> |
---|
606 | <p>For copying between two different versions of Hadoop, one will |
---|
607 | usually use HftpFileSystem. This is a read-only FileSystem, so DistCp |
---|
608 | must be run on the destination cluster (more specifically, on |
---|
609 | TaskTrackers that can write to the destination cluster). Each source is |
---|
610 | specified as <span class="codefrag">hftp://<dfs.http.address>/<path></span> |
---|
611 | (the default <span class="codefrag">dfs.http.address</span> is |
---|
612 | <namenode>:50070).</p> |
---|
613 | <a name="N1026A"></a><a name="Map%2FReduce+and+other+side-effects"></a> |
---|
614 | <h3 class="h4">Map/Reduce and other side-effects</h3> |
---|
615 | <p>As has been mentioned in the preceding, should a map fail to copy |
---|
616 | one of its inputs, there will be several side-effects.</p> |
---|
617 | <ul> |
---|
618 | |
---|
619 | |
---|
620 | <li>Unless <span class="codefrag">-i</span> is specified, the logs generated by that |
---|
621 | task attempt will be replaced by the previous attempt.</li> |
---|
622 | |
---|
623 | |
---|
624 | <li>Unless <span class="codefrag">-overwrite</span> is specified, files successfully |
---|
625 | copied by a previous map on a re-execution will be marked as |
---|
626 | "skipped".</li> |
---|
627 | |
---|
628 | |
---|
629 | <li>If a map fails <span class="codefrag">mapred.map.max.attempts</span> times, the |
---|
630 | remaining map tasks will be killed (unless <span class="codefrag">-i</span> is |
---|
631 | set).</li> |
---|
632 | |
---|
633 | |
---|
634 | <li>If <span class="codefrag">mapred.speculative.execution</span> is set set |
---|
635 | <span class="codefrag">final</span> and <span class="codefrag">true</span>, the result of the copy is |
---|
636 | undefined.</li> |
---|
637 | |
---|
638 | |
---|
639 | </ul> |
---|
640 | </div> <!-- Appendix --> |
---|
641 | |
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642 | |
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643 | </div> |
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644 | <!--+ |
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645 | |end content |
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646 | +--> |
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647 | <div class="clearboth"> </div> |
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648 | </div> |
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649 | <div id="footer"> |
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650 | <!--+ |
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651 | |start bottomstrip |
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653 | <div class="lastmodified"> |
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654 | <script type="text/javascript"><!-- |
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655 | document.write("Last Published: " + document.lastModified); |
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656 | // --></script> |
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657 | </div> |
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658 | <div class="copyright"> |
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659 | Copyright © |
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660 | 2008 <a href="http://www.apache.org/licenses/">The Apache Software Foundation.</a> |
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661 | </div> |
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