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From "Colin Patrick McCabe (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-4960) Unnecessary .meta seeks even when skip checksum is true
Date Wed, 10 Jul 2013 20:11:49 GMT

    [ https://issues.apache.org/jira/browse/HDFS-4960?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13705000#comment-13705000
] 

Colin Patrick McCabe commented on HDFS-4960:
--------------------------------------------

bq. Would that cache the meta header Colin?

No.  We would still read it each time.

bq. Varun, Colin gave me crash-course offline on his option #1 above caching the meta data
header for files in FileInputStreamCache; I can hack up patch when you want something to try...

This seems like the way to go for addressing the concerns in this JIRA.  Not a huge optimization
but it's easy to do.
                
> Unnecessary .meta seeks even when skip checksum is true
> -------------------------------------------------------
>
>                 Key: HDFS-4960
>                 URL: https://issues.apache.org/jira/browse/HDFS-4960
>             Project: Hadoop HDFS
>          Issue Type: Bug
>    Affects Versions: 3.0.0, 2.1.0-beta
>            Reporter: Varun Sharma
>            Assignee: Varun Sharma
>         Attachments: 4960-branch2.patch, 4960-trunk.patch
>
>
> While attempting to benchmark an HBase + Hadoop 2.0 setup on SSDs, we found unnecessary
seeks into .meta files, each seek was a 7 byte read at the head of the file - this attempts
to validate the version #. Since the client is requesting no-checksum, we should not be needing
to touch the .meta file at all.
> Since the purpose of skip checksum is to also avoid the performance penalty of the extra
seek, we should not be seeking into .meta if skip checksum is true

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