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https://issues.apache.org/jira/browse/MAPREDUCE-5907?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16360698#comment-16360698
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Steve Loughran commented on MAPREDUCE-5907:
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Catching up on this after spending many weeks in HADOOP-13786
* Performance boosts with s3 are moot because both the v1 and v2 algorithms are
unreliable with raw s3, and slow even with a consistency layer; it's obsolete.
* none of the other stores *currently* do anything with the listFiles
accordingly, we don't see any immediate benefit in moving to the API, But
if/when someone does implement the listFiles call in a store which also offers
consistent metadata and O(1) renames, then this will help, especially in v1,
where the listing is done in the serialized phase of job commit
> Improve getSplits() performance for fs implementations that can utilize
> performance gains from recursive listing
> ----------------------------------------------------------------------------------------------------------------
>
> Key: MAPREDUCE-5907
> URL: https://issues.apache.org/jira/browse/MAPREDUCE-5907
> Project: Hadoop Map/Reduce
> Issue Type: Improvement
> Components: client
> Affects Versions: 2.4.0
> Reporter: Sumit Kumar
> Assignee: Sumit Kumar
> Priority: Major
> Labels: BB2015-05-TBR
> Attachments: MAPREDUCE-5907-2.patch, MAPREDUCE-5907-3.patch,
> MAPREDUCE-5907.patch
>
>
> FileInputFormat (both mapreduce and mapred implementations) use recursive
> listing while calculating splits. They however do this by doing listing level
> by level. That means to discover files in /foo/bar means they do listing at
> /foo/bar first to get the immediate children, then make the same call on all
> immediate children for /foo/bar to discover their immediate children and so
> on. This doesn't scale well for object store based fs implementations like s3
> and swift because every listStatus call ends up being a webservice call to
> backend. In cases where large number of files are considered for input, this
> makes getSplits() call slow.
> This patch adds a new set of recursive list apis that gives opportunity to
> the fs implementations to optimize. The behavior remains the same for other
> implementations (that is a default implementation is provided for other fs so
> they don't have to implement anything new). However for objectstore based fs
> implementations it provides a simple change to include recursive flag as true
> (as shown in the patch) to improve listing performance.
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