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From "David Mollitor (Jira)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-13157) Do Not Remove Blocks Sequentially During Decommission
Date Fri, 30 Aug 2019 13:32:00 GMT

    [ https://issues.apache.org/jira/browse/HDFS-13157?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16919546#comment-16919546

David Mollitor commented on HDFS-13157:

I'm pretty sure that it is the case the the decommissioning node will be in 1/3 of the replications. 
Making it more distributed will be a much bigger lift.  At the very least I can round-robin
the blocks based on drives.

> 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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