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https://issues.apache.org/jira/browse/HDFS-13156?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16647463#comment-16647463
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Vinayakumar B commented on HDFS-13156:
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+1
> HDFS Block Placement Policy - Client Local Rack
> -----------------------------------------------
>
> Key: HDFS-13156
> URL: https://issues.apache.org/jira/browse/HDFS-13156
> Project: Hadoop HDFS
> Issue Type: Improvement
> Components: documentation
> Affects Versions: 2.9.0, 3.2.0, 3.1.1
> Reporter: BELUGA BEHR
> Assignee: Ayush Saxena
> Priority: Minor
> Attachments: HDFS-13156-01.patch
>
>
> {quote}For the common case, when the replication factor is three, HDFS’s
> placement policy is to put one replica on the local machine if the writer is
> on a datanode, otherwise on a random datanode, another replica on a node in a
> different (remote) rack, and the last on a different node in the same remote
> rack.
> {quote}
> [https://hadoop.apache.org/docs/stable/hadoop-project-dist/hadoop-hdfs/HdfsDesign.html#Replica_Placement:_The_First_Baby_Steps]
> Having just looked over the Default Block Placement code, the way I
> understand this, is that, there are three basic scenarios:
> # HDFS client is running on a datanode inside the cluster
> # HDFS client is running on a node outside the cluster
> # HDFS client is running on a non-datanode inside the cluster
> The documentation is ambiguous concerning the third scenario. Please correct
> me if I'm wrong, but the way I understand the code, if there is an HDFS
> client inside the cluster, but it is not on a datanode, the first block will
> be placed on a datanode within the set of datanodes available on the local
> rack and not simply on any _random datanode_ from the set of all datanodes in
> the cluster.
> That is to say, if one rack has an HDFS Sink Flume Agent on a dedicated node,
> I should expect that every first block will be written to a _random datanode_
> on the same rack as the HDFS Flume agent, assuming the network topology
> script is written to include this Flume node.
> If that is correct, can the documentation be updated to include this third
> common scenario?
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