bz2 input files are handled by FileInputFormat+LineRecordReader. In LineRecordReader, bz2 specific compressed input stream is created to iterate over records. After every new creation, the stream points to the beginning of next data block. The logic to find the beginning of next block depends on start of the split. The search begins at 10 bytes behind the start of split. If the first search creates input stream whose position is before or at start of split, next block beginning is sought (assuming that the record reader for previous split would have already iterated over the the data block in which current start of split lies). If the split start is just at the byte where a newly created stream is positioned (start of data block), attempt is made to find beginning of next data block. This doesn't seem correct because this will result in jumping a whole block and will result in missing records.