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https://issues.apache.org/jira/browse/HDFS-9090?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14791410#comment-14791410
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Walter Su commented on HDFS-9090:
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bq. The placement policy in the erasure coding branch achieves the goal of
spreading the data across racks.
It still could burden local racks where the 10 storm nodes locate on. Maybe you
can customize a policy. Just override {{chooseTargetInOrder}} with completely
{{chooseRandom}}. But it hurts YARN application's performance. HDFS-4894 or
HDFS-7068 could be very helpful but not implemented yet. Maybe you should take
the advice from [~ste...@apache.org] to use ingest nodes.
> Write hot data on few nodes may cause performance issue
> -------------------------------------------------------
>
> Key: HDFS-9090
> URL: https://issues.apache.org/jira/browse/HDFS-9090
> Project: Hadoop HDFS
> Issue Type: Improvement
> Affects Versions: 2.3.0
> Reporter: He Tianyi
> Assignee: He Tianyi
>
> (I am not sure whether this should be reported as BUG, feel free to modify
> this)
> Current block placement policy makes best effort to guarantee first replica
> on local node whenever possible.
> Consider the following scenario:
> 1. There are 500 datanodes across plenty of racks,
> 2. Raw user action log (just an example) are being written only on 10 nodes,
> which also have datanode deployed locally,
> 3. Then, before any balance, all these logs will have at least one replica in
> 10 nodes, implying one thirds data read on these log will be served by these
> 10 nodes if repl factor is 3, performance suffers.
> I propose to solve this scenario by introducing a configuration entry for
> client to disable arbitrary level of write locality.
> Then we can either (A) add local nodes to excludedNodes, or (B) tell NameNode
> the locality we prefer.
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