When I try this.. I get an error cat: Unable to write to output stream. Are these permissions issue How do i resolve this? THanks
On Wed, Feb 20, 2013 at 12:21 PM, Harsh J <[email protected]> wrote: > No problem JM, I was confused as well. > > AFAIK, there's no shell utility that can let you specify an offset # > of bytes to start off with (similar to skip in dd?), but that can be > done from the FS API. > > On Thu, Feb 21, 2013 at 1:14 AM, Jean-Marc Spaggiari > <[email protected]> wrote: > > Hi Harsh, > > > > My bad. > > > > I read the example quickly and I don't know why I tought you used tail > > and not head. > > > > head will work perfectly. But tail will not since it will need to read > > the entier file. My comment was for tail, not for head, and therefore > > not application to the example you gave. > > > > > > hadoop fs -cat 100-byte-dfs-file | tail -c 5 > 5-byte-local-file > > > > Will have to download the entire file. > > > > Is there a way to "jump" into a certain position in a file and "cat" > from there? > > > > JM > > > > 2013/2/20, Harsh J <[email protected]>: > >> Hi JM, > >> > >> I am not sure how "dangerous" it is, since we're using a pipe here, > >> and as you yourself note, it will only last as long as the last bytes > >> have been got and then terminate. > >> > >> The -cat process will terminate because the > >> process we're piping to will terminate first after it reaches its goal > >> of -c <N bytes>; so certainly the "-cat" program will not fetch the > >> whole file down but it may fetch a few bytes extra over communication > >> due to use of read buffers (the extra data won't be put into the target > >> file, and get discarded). > >> > >> We can try it out and observe the "clienttrace" logged > >> at the DN at the end of the -cat's read. Here's an example: > >> > >> I wrote a 1.6~ MB file into a file called "foo.jar", see "bytes" > >> below, its ~1.58 MB: > >> > >> 2013-02-20 23:55:19,777 INFO > >> org.apache.hadoop.hdfs.server.datanode.DataNode.clienttrace: src: > >> /127.0.0.1:58785, dest: /127.0.0.1:50010, bytes: 1658314, op: > >> HDFS_WRITE, cliID: DFSClient_NONMAPREDUCE_915204057_1, offset: 0, > >> srvID: DS-1092147940-192.168.2.1-50010-1349279636946, blockid: > >> BP-1461691939-192.168.2.1-1349279623549:blk_2568668834545125596_73870, > >> duration: 192289000 > >> > >> I ran the command "hadoop fs -cat foo.jar | head -c 5 > foo.xml" to > >> store first 5 bytes onto a local file: > >> > >> Asserting that post command we get 5 bytes: > >> ➜ ~ wc -c foo.xml > >> 5 foo.xml > >> > >> Asserting that DN didn't IO-read the whole file, see the read op below > >> and its "bytes" parameter, its only about 193 KB, not the whole block > >> of 1.58 MB we wrote earlier: > >> > >> 2013-02-21 00:01:32,437 INFO > >> org.apache.hadoop.hdfs.server.datanode.DataNode.clienttrace: src: > >> /127.0.0.1:50010, dest: /127.0.0.1:58802, bytes: 198144, op: > >> HDFS_READ, cliID: DFSClient_NONMAPREDUCE_-1698829178_1, offset: 0, > >> srvID: DS-1092147940-192.168.2.1-50010-1349279636946, blockid: > >> BP-1461691939-192.168.2.1-1349279623549:blk_2568668834545125596_73870, > >> duration: 19207000 > >> > >> I don't see how this is anymore dangerous than doing a > >> -copyToLocal/-get, which retrieves the whole file anyway? > >> > >> On Wed, Feb 20, 2013 at 9:25 PM, Jean-Marc Spaggiari > >> <[email protected]> wrote: > >>> But be careful. > >>> > >>> hadoop fs -cat will retrieve the entire file and last only when it > >>> will have retrieve the last bytes you are looking for. > >>> > >>> If your file is many GB big, it will take a lot of time for this > >>> command to complete and will put some pressure on your network. > >>> > >>> JM > >>> > >>> 2013/2/19, jamal sasha <[email protected]>: > >>>> Awesome thanks :) > >>>> > >>>> > >>>> On Tue, Feb 19, 2013 at 2:14 PM, Harsh J <[email protected]> wrote: > >>>> > >>>>> You can instead use 'fs -cat' and the 'head' coreutil, as one > example: > >>>>> > >>>>> hadoop fs -cat 100-byte-dfs-file | head -c 5 > 5-byte-local-file > >>>>> > >>>>> On Wed, Feb 20, 2013 at 3:38 AM, jamal sasha <[email protected]> > >>>>> wrote: > >>>>> > Hi, > >>>>> > I was wondering in the following command: > >>>>> > > >>>>> > bin/hadoop dfs -copyToLocal hdfspath localpath > >>>>> > can we have specify to copy not full but like xMB's of file to > local > >>>>> drive? > >>>>> > > >>>>> > Is something like this possible > >>>>> > Thanks > >>>>> > Jamal > >>>>> > >>>>> > >>>>> > >>>>> -- > >>>>> Harsh J > >>>>> > >>>> > >> > >> > >> > >> -- > >> Harsh J > >> > > > > -- > Harsh J >
