[ http://issues.apache.org/jira/browse/HADOOP-331?page=comments#action_12442096 ] Bryan Pendleton commented on HADOOP-331: ----------------------------------------
Just have to weigh in, as I always do, with the full-disk and multi-disk viewpoint. It would be nice if, either there's awareness of the disk filling up (set spill size down dynamically if disk free space is less than the default spill size), or if, instead of a single output, there was one output written per output directory. Of course, separate spills should be distributed across different output dirs in some way as they are now. For huge jobs, every bit of space counts, and avoiding failure through simple means can have a big impact on throughput. > map outputs should be written to a single output file with an index > ------------------------------------------------------------------- > > Key: HADOOP-331 > URL: http://issues.apache.org/jira/browse/HADOOP-331 > Project: Hadoop > Issue Type: Improvement > Components: mapred > Affects Versions: 0.3.2 > Reporter: eric baldeschwieler > Assigned To: Devaraj Das > > The current strategy of writing a file per target map is consuming a lot of > unused buffer space (causing out of memory crashes) and puts a lot of burden > on the FS (many opens, inodes used, etc). > I propose that we write a single file containing all output and also write an > index file IDing which byte range in the file goes to each reduce. This will > remove the issue of buffer waste, address scaling issues with number of open > files and generally set us up better for scaling. It will also have > advantages with very small inputs, since the buffer cache will reduce the > number of seeks needed and the data serving node can open a single file and > just keep it open rather than needing to do directory and open ops on every > request. > The only issue I see is that in cases where the task output is substantiallyu > larger than its input, we may need to spill multiple times. In this case, we > can do a merge after all spills are complete (or during the final spill). -- 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
