[MAPREDUCE-2094] LineRecordReader should not seek into non-splittable, compressed streams.

• Type: Bug
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Fix Version/s: 2.8.0, 3.0.0-alpha1
• Component/s: task
• Hadoop Flags: Reviewed
• Assignee: Niels Basjes
• Reporter: Niels Basjes
• Created: 28/Sep/10 09:40
• Updated: 30/Aug/16 01:20
• Resolved: 08/May/15 21:36

Description

When implementing a custom derivative of FileInputFormat we ran into the effect that a large Gzipped input file would be processed several times.

A near 1GiB file would be processed around 36 times in its entirety. Thus producing garbage results and taking up a lot more CPU time than needed.

It took a while to figure out and what we found is that the default implementation of the isSplittable method in org.apache.hadoop.mapreduce.lib.input.FileInputFormat is simply "return true;".

This is a very unsafe default and is in contradiction with the JavaDoc of the method which states: "Is the given filename splitable? Usually, true, but if the file is stream compressed, it will not be. " . The actual implementation effectively does "Is the given filename splitable? Always true, even if the file is stream compressed using an unsplittable compression codec. "

For our situation (where we always have Gzipped input) we took the easy way out and simply implemented an isSplittable in our class that does "return false; "

Now there are essentially 3 ways I can think of for fixing this (in order of what I would find preferable):

1. Implement something that looks at the used compression of the file (i.e. do migrate the implementation from TextInputFormat to FileInputFormat). This would make the method do what the JavaDoc describes.
2. "Force" developers to think about it and make this method abstract.
3. Use a "safe" default (i.e. return false)

Issue Links

• relates to: HADOOP-6901 Parsing large compressed files with HADOOP-1722 spawns multiple mappers per file