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From "Todd Lipcon (Commented) (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-3290) Use a better local directory layout for the datanode
Date Wed, 18 Apr 2012 21:46:40 GMT

    [ https://issues.apache.org/jira/browse/HDFS-3290?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13256999#comment-13256999
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Todd Lipcon commented on HDFS-3290:
-----------------------------------

It doesn't do a "search" for a block. The DN keeps the block map in memory.

But, I do think this is a good idea, as it will make it easier in the future to avoid having
to keep the block map in memory on the DNs.
                
> Use a better local directory layout for the datanode
> ----------------------------------------------------
>
>                 Key: HDFS-3290
>                 URL: https://issues.apache.org/jira/browse/HDFS-3290
>             Project: Hadoop HDFS
>          Issue Type: Improvement
>    Affects Versions: 0.23.0
>            Reporter: Colin Patrick McCabe
>            Assignee: Colin Patrick McCabe
>            Priority: Minor
>
> When the HDFS DataNode stores chunks in a local directory, it currently puts all of the
chunk files into either one big directory, or a collection of directories.  However, there
is no way to know which directory a given block will end up in, given its ID.  As the number
of files increases, this does not scale well.
> Similar to the git version control system, HDFS should create a few different top level
directories keyed off of a few bits in the chunk ID.  Git uses 8 bits.  This substantially
cuts down on the number of chunk files in the same directory and gives increased performance,
while not compromising O(1) lookup of chunks.

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