Having defined a recorddelimiter of multiple bytes in a new InputFileFormat sometimes has the effect of skipping records from the input.
This happens when the input splits are split off just after a recordseparator. Starting point for the next split would be non zero and skipFirstLine would be true. A seek into the file is done to start - 1 and the text until the first recorddelimiter is ignored (due to the presumption that this record is already handled by the previous maptask). Since the re ord delimiter is multibyte the seek only got the last byte of the delimiter into scope and its not recognized as a full delimiter. So the text is skipped until the next delimiter (ignoring a full record!!)