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https://issues.apache.org/jira/browse/HDFS-13157?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16665607#comment-16665607
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Mark Sloan commented on HDFS-13157:
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Has anyone looked at this?
It seems like a random / round robin ordering of the block scheduling for
removal over the available data volumes would allow for significantly better
throughput and processes that don't take days/weeks to complete at the speed of
a single data volume at a time.
> 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: BELUGA BEHR
> 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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