[jira] [Commented] (HADOOP-13208) listFiles(recursive=true) to do a bulk listObjects instead of walking the pseudo-tree of directories

["Steve Loughran (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/HADOOP-13208?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15302913#comment-15302913 ] 

Steve Loughran commented on HADOOP-13208:
-----------------------------------------

One little detail, nothing has been written down about how listFiles() really works, there are no FS contract tests. HADOOP-13207 covers this.

> listFiles(recursive=true) to do a bulk listObjects instead of walking the pseudo-tree of directories
> ----------------------------------------------------------------------------------------------------
>
>                 Key: HADOOP-13208
>                 URL: https://issues.apache.org/jira/browse/HADOOP-13208
>             Project: Hadoop Common
>          Issue Type: Sub-task
>          Components: fs/s3
>    Affects Versions: 2.8.0
>            Reporter: Steve Loughran
>            Assignee: Steve Loughran
>            Priority: Minor
>   Original Estimate: 24h
>  Remaining Estimate: 24h
>
> A major cost in split calculation against object stores turns out be listing the directory tree itself. That's because against S3, it takes S3A two HEADs and two lists to list the content of any directory path (2 HEADs + 1 list for getFileStatus(); the next list to query the contents).
> Listing a directory could be improved slightly by combining the final two listings. However, a listing of a directory tree will still be O(directories). In contrast, a recursive {{listFiles()}} operation should be implementable by a bulk listing of all descendant paths; one List operation per thousand descendants. 
> As the result of this call is an iterator, the ongoing listing can be implemented within the iterator itself



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