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https://issues.apache.org/jira/browse/HADOOP-5734?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12704282#action_12704282
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Hudson commented on HADOOP-5734:
--------------------------------
Integrated in Hadoop-trunk #821 (See
[http://hudson.zones.apache.org/hudson/job/Hadoop-trunk/821/])
. Correct block placement policy description in HDFS Design document.
Contributed by Konstantin Boudnik.
> HDFS architecture documentation describes outdated placement policy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: dfs, documentation
> Affects Versions: 0.20.0
> Reporter: Konstantin Boudnik
> Assignee: Konstantin Boudnik
> Priority: Minor
> Fix For: 0.21.0
>
> Attachments: HADOOP-5734.patch
>
> Time Spent: 2h
> Remaining Estimate: 0h
>
> 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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