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From "Colin Patrick McCabe (JIRA)" <j...@apache.org>
Subject [jira] [Updated] (HDFS-4461) DirectoryScanner: volume path prefix takes up memory for every block that is scanned
Date Fri, 01 Feb 2013 00:55:12 GMT

     [ https://issues.apache.org/jira/browse/HDFS-4461?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Colin Patrick McCabe updated HDFS-4461:
---------------------------------------

    Attachment: memory-analysis.png

here's a before vs. after picture of a memory analysis.  you can see that in the "after" picture,
we are no longer storing the path prefix twice per block in the {{ScanInfo}} class.
                
> DirectoryScanner: volume path prefix takes up memory for every block that is scanned

> -------------------------------------------------------------------------------------
>
>                 Key: HDFS-4461
>                 URL: https://issues.apache.org/jira/browse/HDFS-4461
>             Project: Hadoop HDFS
>          Issue Type: Improvement
>    Affects Versions: 2.0.3-alpha
>            Reporter: Colin Patrick McCabe
>            Assignee: Colin Patrick McCabe
>            Priority: Minor
>         Attachments: 002.patch, memory-analysis.png
>
>
> In the {{DirectoryScanner}}, we create a class {{ScanInfo}} for every block.  This object
contains two File objects-- one for the metadata file, and one for the block file.  Since
those File objects contain full paths, users who pick a lengthly path for their volume roots
will end up using an extra N_blocks * path_prefix bytes per block scanned.  We also don't
really need to store File objects-- storing strings and then creating File objects as needed
would be cheaper.  This has been causing out-of-memory conditions for users who pick such
long volume paths.

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