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From "Kihwal Lee (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-7174) Support for more efficient large directories
Date Tue, 30 Sep 2014 22:46:34 GMT

    [ https://issues.apache.org/jira/browse/HDFS-7174?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14153924#comment-14153924
] 

Kihwal Lee commented on HDFS-7174:
----------------------------------

The threshold is set to 50,000 In the patch.  This will make the existing listing semantics
is preserved for regular applications, while special applications can benefit from the performance
improvement.  It converts back to the normal list when it hits 90% the threshold.

> Support for more efficient large directories
> --------------------------------------------
>
>                 Key: HDFS-7174
>                 URL: https://issues.apache.org/jira/browse/HDFS-7174
>             Project: Hadoop HDFS
>          Issue Type: Improvement
>            Reporter: Kihwal Lee
>            Assignee: Kihwal Lee
>            Priority: Critical
>         Attachments: HDFS-7174.patch
>
>
> When the number of children under a directory grows very large, insertion becomes very
costly.  E.g. creating 1M entries takes 10s of minutes.  This is because the complexity of
an insertion is O\(n\). As the size of a list grows, the overhead grows n^2. (integral of
linear function).  It also causes allocations and copies of big arrays.



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