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https://issues.apache.org/jira/browse/HDFS-13157?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16919628#comment-16919628
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Stephen O'Donnell commented on HDFS-13157:
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This is a very interesting observation. The DatanodeAdminManager writes the
blocks to be replicated into the replication queue, and then it is processed
like a queue, in order, where work is then allocated to the datanodes. So it
seems you are correct, in that the blocks will be replicated disk by disk.
{quote}
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.
{quote}
It is also worth keeping in mind, that a node in decommissioning state will not
receive any new writes and reads are only directed to it as a last resort (ie
no other replica available). Therefore the load on a decommission node would
normally be less, so it should be able to replicate more. Also each DN can only
have a set number of inflight transfers at once. So if the decommissioning node
was not able to keep up, I think more of its blocks would get scheduled on
other nodes as they clear their work queue more quickly, but I am not certain
on that.
One observation I had when looking at decommissioning, is that the NN write
lock is held for the entire time it takes to process the blocks on a given DN
on the first pass. This time can be several seconds on a node with a lot of
blocks. HDFS-10477 observed the same behaviour for recommission and made a
change to drop the write lock after processing each storage rather that holding
it for all storages. I wonder if shuffling the iterators would make a change
like that for starting the decommission impossible?
> 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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