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Date: Fri, 27 Jun 2008 12:55:45 -0700 (PDT)
From: "Robert Chansler (JIRA)" <jira@apache.org>
To: core-dev@hadoop.apache.org
Subject: [jira] Updated: (HADOOP-1702) Reduce buffer copies when data is
 written to DFS
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     [ https://issues.apache.org/jira/browse/HADOOP-1702?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Robert Chansler updated HADOOP-1702:
------------------------------------

    Release Note: Reduced buffer copies as data is written to HDFS. The order of sending data bytes and control information has changed, but this will not be observed by client applications.  (was: Reduce buffer copies when data is written to DFS. DataNode takes 30% less CPU. As a result, the format of data DFSClient sends changed and is incompatible with previous clients.)
    Hadoop Flags: [Incompatible change, Reviewed]  (was: [Reviewed, Incompatible change])

> Reduce buffer copies when data is written to DFS
> ------------------------------------------------
>
>                 Key: HADOOP-1702
>                 URL: https://issues.apache.org/jira/browse/HADOOP-1702
>             Project: Hadoop Core
>          Issue Type: Bug
>          Components: dfs
>    Affects Versions: 0.14.0
>            Reporter: Raghu Angadi
>            Assignee: Raghu Angadi
>             Fix For: 0.18.0
>
>         Attachments: HADOOP-1702.patch, HADOOP-1702.patch, HADOOP-1702.patch, HADOOP-1702.patch, HADOOP-1702.patch, HADOOP-1702.patch, HADOOP-1702.patch, HADOOP-1702.patch, HADOOP-1702.patch
>
>
> HADOOP-1649 adds extra buffering to improve write performance.  The following diagram shows buffers as pointed by (numbers). Each eatra buffer adds an extra copy since most of our read()/write()s match the io.bytes.per.checksum, which is much smaller than buffer size.
> {noformat}
>        (1)                 (2)          (3)                 (5)
>    +---||----[ CLIENT ]---||----<>-----||---[ DATANODE ]---||--<>-> to Mirror  
>    | (buffer)                  (socket)           |  (4)
>    |                                              +--||--+
>  =====                                                    |
>  =====                                                  =====
>  (disk)                                                 =====
> {noformat}
> Currently loops that read and write block data, handle one checksum chunk at a time. By reading multiple chunks at a time, we can remove buffers (1), (2), (3), and (5). 
> Similarly some copies can be reduced when clients read data from the DFS.

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