[ http://issues.apache.org/jira/browse/HADOOP-54?page=comments#action_12421380 ] Bryan Pendleton commented on HADOOP-54: ---------------------------------------
Another feature that might be useful: include an (optional) un-compressed copy of the *last* key in a given compression spill. Why? Because, for sorted SequenceFiles (specifically, for the data file of a MapFile), when seeking through to find a given key/value, knowing the last key in a given chunk allows skipping of decompressing the entire key array. It should, of course, be optional, both because keys potentially be large, and because SequenceFiles aren't all sorted. But, in the MapFile case, it could reduce the cost of finding a hit during lookups. Might also be useful to try to do some DFS block-aligning, to avoid block requests and CRC calculations for data that's not really going to be used. That sounds like it might be tricky to get, though, because default GFS blocks are so large, and we're probably talking about much smaller compression chunks. Does DFS have variable-length block writing yet? > 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: Michel Tourn > Fix For: 0.5.0 > > > 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
