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From "stack (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-4960) Unnecessary .meta seeks even when skip checksum is true
Date Wed, 10 Jul 2013 18:09:49 GMT

    [ https://issues.apache.org/jira/browse/HDFS-4960?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13704848#comment-13704848

stack commented on HDFS-4960:

bq. You could try increasing the size of the FileInputStreamCache in HDFS by setting dfs.client.read.shortcircuit.streams.cache.size
to something bigger than 100.

Would that cache the meta header Colin?

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...
> 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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