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https://issues.apache.org/jira/browse/HADOOP-14411?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16012626#comment-16012626
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Akira Ajisaka commented on HADOOP-14411:
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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 HDFS architecture documentation describes outdated placement policy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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