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From "Hairong Kuang (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-2172) PositionCache was removed from FSDataInputStream, causes extremely bad MapFile performance
Date Thu, 08 Nov 2007 21:41:50 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-2172?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12541144
] 

Hairong Kuang commented on HADOOP-2172:
---------------------------------------

LocalFileSystem is a ChecksumFileSystem. So the file position is cached in FSInputChecker.
I still do not understand why taking off PositionCache causes such a huge performance degragation.

> PositionCache was removed from FSDataInputStream, causes extremely bad MapFile performance
> ------------------------------------------------------------------------------------------
>
>                 Key: HADOOP-2172
>                 URL: https://issues.apache.org/jira/browse/HADOOP-2172
>             Project: Hadoop
>          Issue Type: Bug
>          Components: fs
>    Affects Versions: 0.14.3, 0.15.0
>            Reporter: Johan Oskarsson
>            Assignee: Johan Oskarsson
>            Priority: Blocker
>             Fix For: 0.15.1
>
>         Attachments: HADOOP-2172-2.patch, positioncache-v1.patch
>
>
> The PositionCache in FSDataInputStream seems to have been removed in HADOOP-1470. This
causes for example MapFile.get usage to be  extremely slow as the file position isn't cached
in memory.

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