[jira] Created: (MAPREDUCE-1892) RaidNode can identify processed files with lesser memory usage

["Ramkumar Vadali (JIRA)" <ji...@apache.org>]

RaidNode can identify processed files with lesser memory usage
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                 Key: MAPREDUCE-1892
                 URL: https://issues.apache.org/jira/browse/MAPREDUCE-1892
             Project: Hadoop Map/Reduce
          Issue Type: Improvement
          Components: contrib/raid
            Reporter: Ramkumar Vadali


The RaidNode policy file can have policies that can cover a file more than once. To avoid processing a file multiple times (for RAIDing), RaidNode maintains a list of processed files that is used to avoid duplicate processing attempts.

This is problematic in that a large number of processed files could cause the RaidNode to run out of memory.

This task proposes a better method of detecting processed files. The method is based on the observation that a more selective policy will have a better match with a file name than a less selective one. Specifically, the more selective policy will have a longer common prefix with the file name.

So to detect if a file has already been processed, the RaidNode only needs to maintain a list of processed policies and compare the lengths of the common prefixes. If the file has a longer common prefix with one of the processed policies than with the current policy, it can be assumed to be processed already.

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