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From "liang xie (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-3429) DataNode reads checksums even if client does not need them
Date Sat, 15 Dec 2012 02:06:14 GMT

    [ https://issues.apache.org/jira/browse/HDFS-3429?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13532891#comment-13532891
] 

liang xie commented on HDFS-3429:
---------------------------------

O, [~tlipcon], you missed my words: "without patch"

the strace showed the statistic without patch.  
After applied the patch, i could not see so much meta files be opened
                
> DataNode reads checksums even if client does not need them
> ----------------------------------------------------------
>
>                 Key: HDFS-3429
>                 URL: https://issues.apache.org/jira/browse/HDFS-3429
>             Project: Hadoop HDFS
>          Issue Type: Bug
>          Components: datanode, performance
>    Affects Versions: 2.0.0-alpha
>            Reporter: Todd Lipcon
>            Assignee: Todd Lipcon
>         Attachments: hdfs-3429-0.20.2.patch, hdfs-3429-0.20.2.patch, hdfs-3429.txt, hdfs-3429.txt,
hdfs-3429.txt
>
>
> Currently, even if the client does not want to verify checksums, the datanode reads them
anyway and sends them over the wire. This means that performance improvements like HBase's
application-level checksums don't have much benefit when reading through the datanode, since
the DN is still causing seeks into the checksum file.
> (Credit goes to Dhruba for discovering this - filing on his behalf)

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