MapFile is layered on SequenceFile and permits random-access to sorted data files (typically reduce output) through a parallel index file. This is used widely in Nutch (e.g. at search time for displaying cached pages, incoming links, etc). Such sorted data should benefit from block compression, but the current MapFile API does not support specification of block compression. Also, even if it did, the semantics of SequenceFile methods like seek() and getPosition() are changed under block compression so that MapFile may not work.