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From "Todd Lipcon (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-4461) DirectoryScanner: volume path prefix takes up memory for every block that is scanned
Date Fri, 01 Feb 2013 19:22:14 GMT

    [ https://issues.apache.org/jira/browse/HDFS-4461?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13568989#comment-13568989
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Todd Lipcon commented on HDFS-4461:
-----------------------------------

Looks like we can cut the memory usage in half again -- storing both the metafile path and
the block file path is redundant, since you can always compute the block path from the meta
path by chopping off the "_<genstamp>.meta" prefix.

Suresh -- we routinely see users with millions of replicas per DN now that 48TB+ configurations
have become commodity. Sure, we should also encourage users to use things like HAR to coalesce
into larger blocks, but easy wins on DN memory usage are a no-brainer IMO.
                
> 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: HDFS-4461.002.patch, HDFS-4461.003.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 would be a nice efficiency improvement.

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