[ http://issues.apache.org/jira/browse/HADOOP-54?page=comments#action_12422393 ] Doug Cutting commented on HADOOP-54: ------------------------------------
Just to be clear, delta compression means something different to me than zero-compression. The former represents a lists of integers with their differences, the latter elides leading zeros in integers. They're not exclusive. A sorted list of integers is smaller when delta compressed and zero-compressed. A random list of integers is probably not helped by delta compression, but is helped by zero compression. If values are in a narrow range, then delta compression may help. Thus it may be useful for lists of key lengths and value lengths. You provide some benchmarks showing the advantage of zero compression for random sequences. Eric said delta compression, but I think he meant zero compression. I agree that we should use zero compression everywhere. The only question is if we should also use delta compression anywhere. > SequenceFile should compress blocks, not individual entries > ----------------------------------------------------------- > > Key: HADOOP-54 > URL: http://issues.apache.org/jira/browse/HADOOP-54 > Project: Hadoop > Issue Type: Improvement > Components: io > Affects Versions: 0.2.0 > Reporter: Doug Cutting > Assigned To: Arun C Murthy > Fix For: 0.5.0 > > Attachments: VIntCompressionResults.txt > > > SequenceFile will optionally compress individual values. But both > compression and performance would be much better if sequences of keys and > values are compressed together. Sync marks should only be placed between > blocks. This will require some changes to MapFile too, so that all file > positions stored there are the positions of blocks, not entries within > blocks. Probably this can be accomplished by adding a > getBlockStartPosition() method to SequenceFile.Writer. -- This message is automatically generated by JIRA. - If you think it was sent incorrectly contact one of the administrators: http://issues.apache.org/jira/secure/Administrators.jspa - For more information on JIRA, see: http://www.atlassian.com/software/jira
