[jira] Issue Comment Edited: (HADOOP-5734) HDFS architecture documentation describes outdated placement policy

["Konstantin Boudnik (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/HADOOP-5734?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12702200#action_12702200 ] 

Konstantin Boudnik edited comment on HADOOP-5734 at 4/23/09 6:46 PM:
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The fix is clearly a minor modification of the placement logic's description. 
It doesn't affect any code but the documentation template and the JavaDoc of the ReplicationTargetChooser class, which is invisible anywhere but in the source code of the class.

However, as I'm a very new to the project I'd appreciate if someone can make a quick review of at the patch to the internal javadoc to make sure it is "accurate, sufficient for future maintainability".


      was (Author: cos):
    The fix is clearly a minor modification of the placement logic's description. 
It doesn't affect any code but the documentation template and the JavaDoc of the ReplicationTargetChooser class, which is invisible anywhere but in the source code of the class 
  
> HDFS architecture documentation describes outdated placement policy
> -------------------------------------------------------------------
>
>                 Key: HADOOP-5734
>                 URL: https://issues.apache.org/jira/browse/HADOOP-5734
>             Project: Hadoop Core
>          Issue Type: Bug
>          Components: documentation
>    Affects Versions: 0.20.0
>            Reporter: Konstantin Boudnik
>            Priority: Minor
>         Attachments: HADOOP-5734.patch
>
>
> The "Replica Placement: The First Baby Steps" section of HDFS architecture document states:
> "...
> For the common case, when the replication factor is three, HDFS's placement policy is to put one replica on one node in the local rack, another on a different node in the local rack, and the last on a different node in a different rack. This policy cuts the inter-rack write traffic which generally improves write performance.
> ..."
> However, according to the ReplicationTargetChooser.chooseTarger()'s code the actual logic is to put the second replica on a different rack as well as the third replica. So you have two replicas located on a different nodes of remote rack and one (initial replica) on the local rack's node. Thus, the sentence should say something like this:
> "For the common case, when the replication factor is three, HDFS's placement policy is to put one replica on one node in the local rack, another on a node in a different (remote) rack, and the last on a different node in the same remote rack. This policy cuts the inter-rack write traffic which generally improves write performance."

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