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https://issues.apache.org/jira/browse/HDFS-9090?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14791457#comment-14791457
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He Tianyi commented on HDFS-9090:
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Not quite sure but I do think perhaps this is perpendicular with
{{BlockPlacementPolicy}}.
Assume that HDFS-7068 is implemented. In this case, one can configure
{{BlockPlacementPolicy}} for specified INode. It is certain that write
operation under particular directory can be enforced to scatter data across the
cluster.
But, given that {{BlockPlacementPolicy}} focuses on where replica should be
located, each identical policy may differentiate to two different versions
(with locality, and without).
That is, we have {{BlockPlacementPolicyDefault}}, then perhaps we need a
{{BlockPlacementPolicyDefaultWithoutWriteLocality}}.
And for a real case, we have {{BlockPlacementPolicyWithMultiDC}}, then perhaps
we also need a {{BlockPlacementPolicyWithMultiDCWithoutWriteLocality}}.
Let alone the latter one could be implemented by just overriding several
methods.
Based on that, how about add one parameter, perhaps named "localityLevel" to
{{chooseTarget}}, then each policy can have their own consideration without
having the burden of implement two versions?
This could also work when multiple policy is not supported.
> 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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