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https://issues.apache.org/jira/browse/HDFS-13157?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16920137#comment-16920137
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Stephen O'Donnell commented on HDFS-13157:
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Another thought - if this problem exists, where replication works predominantly
off one decommissioning disk on the node, then if you decommission X nodes,
does that mean the first node tends to decommission at a faster rate than the
second due to the order the blocks are added to the replication queue? Ie, Node
1's blocks are all added, then node 2's etc. I think we need to dig into how
the replication queue is processed in some more detail to really understand how
blocks are replicated after getting into that queue. There are several limits
in play, such is max-streams, max-streams-hard-limit (which are per DN) and
work multiplier which governs the total blocks that can have replication
in-flight across the entire cluster.
> 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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