title: About Hadoop/HDFS file splitting
body: <p>Want to just confirm on following. Please verify if this is correct:
  1. As per my understanding when we copy a file into HDFS, that is the point when file (assuming its size > 64MB = HDFS block size) is split into multiple chunks and each chunk is stored on different data-nodes.</p>
  
  <ol>
  <li><p>File contents are already split into chunks when file is copied into HDFS and that file-split does not happen at the time of running map job. Map tasks are only scheduled in such a way that they work on each chunk of max. size 64 MB with data-locality (i.e. map task runs on that node which contains the data/chunk)</p></li>
  <li><p>File splitting also happens if file is compressed (gzipped) but MR ensures that each file is processed by just one mapper, i.e. MR will collect all the chunks of gzip file lying at other data nodes and give them all to the single mapper.</p></li>
  <li><p>Same thing as above will happen if we define isSplitable() to return false, i.e. all the chunks of a file will be processed by one mapper running on one machine. MR will read all the chunks of a file from different data-nodes and make them available to a single mapper.</p></li>
  </ol>