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From "Akira Ajisaka (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HADOOP-14411) HDFS architecture documentation describes outdated placement policy
Date Tue, 16 May 2017 15:51:04 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-14411?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16012626#comment-16012626
] 

Akira Ajisaka commented on HADOOP-14411:
----------------------------------------

LGTM, +1

> HDFS architecture documentation describes outdated placement policy
> -------------------------------------------------------------------
>
>                 Key: HADOOP-14411
>                 URL: https://issues.apache.org/jira/browse/HADOOP-14411
>             Project: Hadoop Common
>          Issue Type: Improvement
>            Reporter: dud
>            Assignee: Chen Liang
>            Priority: Minor
>              Labels: documentation
>         Attachments: HADOOP-14411.001.patch, HADOOP-14411.002.patch
>
>
> Hello
> I've noticed inconsistencies in the default block placement policy between what one can
read on several websites and what is stated in the [official documentation|https://git-wip-us.apache.org/repos/asf?p=hadoop.git;a=blob;f=hadoop-hdfs-project/hadoop-hdfs/src/site/markdown/HdfsDesign.md;h=bda53a9c77b41308eef3abf5b478158b2ea03be0;hb=HEAD#l105].
> After digging, I've found out that the proper default placement policy is :
> {code}
>  * The replica placement strategy is that if the writer is on a datanode,
>  * the 1st replica is placed on the local machine, 
>  * otherwise a random datanode. The 2nd replica is placed on a datanode
>  * that is on a different rack. The 3rd replica is placed on a datanode
>  * which is on a different node of the rack as the second replica.
>  {code}
> [source|https://git-wip-us.apache.org/repos/asf?p=hadoop.git;a=blob;f=hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/blockmanagement/BlockPlacementPolicyDefault.java#l45]
> HADOOP-5734 has been opened regarding this mistake several years ago and has eventually
been merged but unfortunately this fix has been overwritten by [~tucu00]  in [SVN commit|https://svn.apache.org/viewvc?view=revision&revision=1425527].
> dud



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