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https://issues.apache.org/jira/browse/HDFS-13157?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16919534#comment-16919534
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David Mollitor commented on HDFS-13157:
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Also... One area of research...
I'm not sure if the Replication Commands are all sent to the one
decommissioning DataNode or if the blocks that needs to be replicated are
examined and the load is spread across all of the DataNodes. Even if that is
the case, if the block replication is 3, and the nodes selected for replication
are random, the decommissioning node will be selected 1/3 of the time for
replication. With that said, perhaps the decommissioning node should be
exempted from replication altogether (for all blocks with replication >1) so
that the load is spread randomly throughout all the DataNodes in the cluster
and do not get concentrated on the decommissioning node.
> Do Not Remove Blocks Sequentially During Decommission
> ------------------------------------------------------
>
> Key: HDFS-13157
> URL: https://issues.apache.org/jira/browse/HDFS-13157
> Project: Hadoop HDFS
> Issue Type: Improvement
> Components: datanode, namenode
> Affects Versions: 3.0.0
> Reporter: David Mollitor
> Assignee: David Mollitor
> Priority: Major
>
> From what I understand of [DataNode
> decommissioning|https://github.com/apache/hadoop/blob/42a1c98597e6dba2e371510a6b2b6b1fb94e4090/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/blockmanagement/DatanodeAdminManager.java]
> it appears that all the blocks are scheduled for removal _in order._. I'm
> not 100% sure what the ordering is exactly, but I think it loops through each
> data volume and schedules each block to be replicated elsewhere. The net
> affect is that during a decommission, all of the DataNode transfer threads
> slam on a single volume until it is cleaned out. At which point, they all
> slam on the next volume, etc.
> Please randomize the block list so that there is a more even distribution
> across all volumes when decommissioning a node.
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