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From "Josh Wills (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CRUNCH-256) SequentialFileNamingScheme should cache the # of files in the target directory after the first read
Date Fri, 23 Aug 2013 03:32:54 GMT

    [ https://issues.apache.org/jira/browse/CRUNCH-256?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13748251#comment-13748251
] 

Josh Wills commented on CRUNCH-256:
-----------------------------------

IIRC, it had to deal w/handling the output of a union operation, i.e., I could have multiple
jobs in a pipeline that were all writing to the same output directory for subsequent processing.
If a target output directory already existed and had files in it, we wanted to first write
the output of the MR job to a temp directory, and then move the output of the job to the target
directory for further processing. I suspect that to make the logic easier, we just implemented
things so that the move was always done, since it didn't seem to cost that much most of the
time.

Obviously this patch is just a bandaid to fix one of the more pernicious problems with this
approach, and we should still see about tackling CRUNCH-252 so that we can eliminate the move
entirely when it's not necessary.
                
> SequentialFileNamingScheme should cache the # of files in the target directory after
the first read
> ---------------------------------------------------------------------------------------------------
>
>                 Key: CRUNCH-256
>                 URL: https://issues.apache.org/jira/browse/CRUNCH-256
>             Project: Crunch
>          Issue Type: Bug
>    Affects Versions: 0.7.0
>            Reporter: Josh Wills
>            Assignee: Josh Wills
>             Fix For: 0.8.0
>
>         Attachments: CRUNCH-256.patch
>
>
> After a job finishes running, the post-job hooks rename the files from a temp output
directory to the target output directory. When we have lots of files, this move can take a
long time, and I traced the performance issue to the fact that SequentialFileNamingScheme
does a listStatus() on the output directory for every file that gets moved. If SequentialFileNamingScheme
just does this check once and then increments an internal counter, we can significantly decrease
the performance overhead involved with the move.

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