Mailing-List: contact common-dev-help@hadoop.apache.org; run by ezmlm
Precedence: bulk
Reply-To: common-dev@hadoop.apache.org
Message-ID: <1849532762.1248038175171.JavaMail.jira@brutus>
Date: Sun, 19 Jul 2009 14:16:15 -0700 (PDT)
From: "Chris Douglas (JIRA)" <jira@apache.org>
To: common-dev@hadoop.apache.org
Subject: [jira] Reopened: (HADOOP-4010) Chaging LineRecordReader algo so
 that it does not need to skip backwards in the stream
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: 7bit


     [ https://issues.apache.org/jira/browse/HADOOP-4010?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Chris Douglas reopened HADOOP-4010:
-----------------------------------


> Chaging LineRecordReader algo so that it does not need to skip backwards in the stream
> --------------------------------------------------------------------------------------
>
>                 Key: HADOOP-4010
>                 URL: https://issues.apache.org/jira/browse/HADOOP-4010
>             Project: Hadoop Common
>          Issue Type: Improvement
>    Affects Versions: 0.19.0
>            Reporter: Abdul Qadeer
>            Assignee: Abdul Qadeer
>             Fix For: 0.21.0
>
>         Attachments: 4010-mapreduce.patch, Hadoop-4010.patch, Hadoop-4010_version2.patch, Hadoop-4010_version3.patch
>
>
> The current algorithm of the LineRecordReader needs to move backwards in the stream (in its constructor) to correctly position itself in the stream.  So it moves back one byte from the start of its split and try to read a record (i.e. a line) and throws that away.  This is so because it is sure that, this line would be taken care of by some other mapper.  This algorithm is difficult and in-efficient if used for compressed stream where data is coming to the LineRecordReader via some codecs. (Although in the current implementation, Hadoop does not split a compressed file and only makes one split from the start to the end of the file and so only one mapper handles it.  We are currently working on BZip2 codecs where splitting is possible to work with Hadoop.  So this proposed change will make it possible to uniformly handle plain as well as compressed stream.)
> In the new algorithm, each mapper always skips its first line because it is sure that, that line would have been read by some other mapper.  So now each mapper must finish its reading at a record boundary which is always beyond its upper split limit.  Due to this change, LineRecordReader does not need to move backwards in the stream.

-- 
This message is automatically generated by JIRA.
-
You can reply to this email to add a comment to the issue online.