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https://issues.apache.org/jira/browse/HDFS-14383?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16808050#comment-16808050
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Karthik Palanisamy commented on HDFS-14383:
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{quote}I suppose this load issue wouldn't occur if you configure datanodes such
that they have some HOT and some COLD volumes.
{quote}
Exactly [~jojochuang]. Problem only with some datanodes which are specific
either COLD or HOT.
IMO - it is typical to see this setup because hardware and resources are
specific to COLD/HOT storage.
> Compute datanode load based on StoragePolicy
> --------------------------------------------
>
> Key: HDFS-14383
> URL: https://issues.apache.org/jira/browse/HDFS-14383
> Project: Hadoop HDFS
> Issue Type: Bug
> Components: hdfs, namenode
> Affects Versions: 2.7.3, 3.1.2
> Reporter: Karthik Palanisamy
> Assignee: Karthik Palanisamy
> Priority: Major
>
> Datanode load check logic needs to be changed because existing computation
> will not consider StoragePolicy.
> DatanodeManager#getInServiceXceiverAverage
> {code}
> public double getInServiceXceiverAverage() {
> double avgLoad = 0;
> final int nodes = getNumDatanodesInService();
> if (nodes != 0) {
> final int xceivers = heartbeatManager
> .getInServiceXceiverCount();
> avgLoad = (double)xceivers/nodes;
> }
> return avgLoad;
> }
> {code}
>
> For example: with 10 nodes (HOT), average 50 xceivers and 90 nodes (COLD)
> with average 10 xceivers the calculated threshold by the NN is 28 (((500 +
> 900)/100)*2), which means those 10 nodes (the whole HOT tier) becomes
> unavailable when the COLD tier nodes are barely in use. Turning this check
> off helps to mitigate this issue, however the
> dfs.namenode.replication.considerLoad helps to "balance" the load of the DNs,
> upon turning it off can lead to situations where specific DNs are
> "overloaded".
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