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From "dhruba borthakur (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-1702) Reduce buffer copies when data is written to DFS
Date Mon, 07 Apr 2008 06:50:24 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-1702?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12586254#action_12586254
] 

dhruba borthakur commented on HADOOP-1702:
------------------------------------------

I would vote for keeping the packetSize fixed at 64K and not make it dependent on any user
defined configuration parameter. 

> 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
>         Attachments: 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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