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https://issues.apache.org/jira/browse/HDFS-16303?focusedWorklogId=682418&page=com.atlassian.jira.plugin.system.issuetabpanels:worklog-tabpanel#worklog-682418
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ASF GitHub Bot logged work on HDFS-16303:
-----------------------------------------

                Author: ASF GitHub Bot
            Created on: 17/Nov/21 07:53
            Start Date: 17/Nov/21 07:53
    Worklog Time Spent: 10m 
      Work Description: hadoop-yetus commented on pull request #3667:
URL: https://github.com/apache/hadoop/pull/3667#issuecomment-971323286


   :broken_heart: **-1 overall**
   
   
   
   
   
   
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[/results-checkstyle-hadoop-hdfs-project_hadoop-hdfs.txt](https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-3667/1/artifact/out/results-checkstyle-hadoop-hdfs-project_hadoop-hdfs.txt)
 |  hadoop-hdfs-project/hadoop-hdfs: The patch generated 5 new + 45 unchanged - 
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   |||| _ Other Tests _ |
   | -1 :x: |  unit  | 395m 51s | 
[/patch-unit-hadoop-hdfs-project_hadoop-hdfs.txt](https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-3667/1/artifact/out/patch-unit-hadoop-hdfs-project_hadoop-hdfs.txt)
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   |  |   | 513m 13s |  |  |
   
   
   | Reason | Tests |
   |-------:|:------|
   | Failed junit tests | hadoop.hdfs.TestDecommissionWithBackoffMonitor |
   |   | hadoop.hdfs.TestHDFSFileSystemContract |
   |   | hadoop.hdfs.web.TestWebHdfsFileSystemContract |
   
   
   | Subsystem | Report/Notes |
   |----------:|:-------------|
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https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-3667/1/artifact/out/Dockerfile
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   | GITHUB PR | https://github.com/apache/hadoop/pull/3667 |
   | Optional Tests | dupname asflicense compile javac javadoc mvninstall 
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   | uname | Linux df4ebb1bd981 4.15.0-143-generic #147-Ubuntu SMP Wed Apr 14 
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   | Build tool | maven |
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Build-1.8.0_292-8u292-b10-0ubuntu1~20.04-b10 |
   |  Test Results | 
https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-3667/1/testReport/ |
   | Max. process+thread count | 1990 (vs. ulimit of 5500) |
   | modules | C: hadoop-hdfs-project/hadoop-hdfs U: 
hadoop-hdfs-project/hadoop-hdfs |
   | Console output | 
https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-3667/1/console |
   | versions | git=2.25.1 maven=3.6.3 spotbugs=4.2.2 |
   | Powered by | Apache Yetus 0.14.0-SNAPSHOT https://yetus.apache.org |
   
   
   This message was automatically generated.
   
   


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Issue Time Tracking
-------------------

    Worklog Id:     (was: 682418)
    Time Spent: 20m  (was: 10m)

> Losing over 100 datanodes in state decommissioning results in full blockage 
> of all datanode decommissioning
> -----------------------------------------------------------------------------------------------------------
>
>                 Key: HDFS-16303
>                 URL: https://issues.apache.org/jira/browse/HDFS-16303
>             Project: Hadoop HDFS
>          Issue Type: Bug
>    Affects Versions: 2.10.1, 3.3.1
>            Reporter: Kevin Wikant
>            Priority: Major
>              Labels: pull-request-available
>          Time Spent: 20m
>  Remaining Estimate: 0h
>
> h2. Impact
> HDFS datanode decommissioning does not make any forward progress. For 
> example, the user adds X datanodes to the "dfs.hosts.exclude" file and all X 
> of those datanodes remain in state decommissioning forever without making any 
> forward progress towards being decommissioned.
> h2. Root Cause
> The HDFS Namenode class "DatanodeAdminManager" is responsible for 
> decommissioning datanodes.
> As per this "hdfs-site" configuration:
> {quote}Config = dfs.namenode.decommission.max.concurrent.tracked.nodes 
>  Default Value = 100
> The maximum number of decommission-in-progress datanodes nodes that will be 
> tracked at one time by the namenode. Tracking a decommission-in-progress 
> datanode consumes additional NN memory proportional to the number of blocks 
> on the datnode. Having a conservative limit reduces the potential impact of 
> decomissioning a large number of nodes at once. A value of 0 means no limit 
> will be enforced.
> {quote}
> The Namenode will only actively track up to 100 datanodes for decommissioning 
> at any given time, as to avoid Namenode memory pressure.
> Looking into the "DatanodeAdminManager" code:
>  * a new datanode is only removed from the "tracked.nodes" set when it 
> finishes decommissioning
>  * a new datanode is only added to the "tracked.nodes" set if there is fewer 
> than 100 datanodes being tracked
> So in the event that there are more than 100 datanodes being decommissioned 
> at a given time, some of those datanodes will not be in the "tracked.nodes" 
> set until 1 or more datanodes in the "tracked.nodes" finishes 
> decommissioning. This is generally not a problem because the datanodes in 
> "tracked.nodes" will eventually finish decommissioning, but there is an edge 
> case where this logic prevents the namenode from making any forward progress 
> towards decommissioning.
> If all 100 datanodes in the "tracked.nodes" are unable to finish 
> decommissioning, then other datanodes (which may be able to be 
> decommissioned) will never get added to "tracked.nodes" and therefore will 
> never get the opportunity to be decommissioned.
> This can occur due the following issue:
> {quote}2021-10-21 12:39:24,048 WARN 
> org.apache.hadoop.hdfs.server.blockmanagement.BlockManager 
> (DatanodeAdminMonitor-0): Node W.X.Y.Z:50010 is dead while in Decommission In 
> Progress. Cannot be safely decommissioned or be in maintenance since there is 
> risk of reduced data durability or data loss. Either restart the failed node 
> or force decommissioning or maintenance by removing, calling refreshNodes, 
> then re-adding to the excludes or host config files.
> {quote}
> If a Datanode is lost while decommissioning (for example if the underlying 
> hardware fails or is lost), then it will remain in state decommissioning 
> forever.
> If 100 or more Datanodes are lost while decommissioning over the Hadoop 
> cluster lifetime, then this is enough to completely fill up the 
> "tracked.nodes" set. With the entire "tracked.nodes" set filled with 
> datanodes that can never finish decommissioning, any datanodes added after 
> this point will never be able to be decommissioned because they will never be 
> added to the "tracked.nodes" set.
> In this scenario:
>  * the "tracked.nodes" set is filled with datanodes which are lost & cannot 
> be recovered (and can never finish decommissioning so they will never be 
> removed from the set)
>  * the actual live datanodes being decommissioned are enqueued waiting to 
> enter the "tracked.nodes" set (and are stuck waiting indefinitely)
> This means that no progress towards decommissioning the live datanodes will 
> be made unless the user takes the following action:
> {quote}Either restart the failed node or force decommissioning or maintenance 
> by removing, calling refreshNodes, then re-adding to the excludes or host 
> config files.
> {quote}
> Ideally, the Namenode should be able to gracefully handle scenarios where the 
> datanodes in the "tracked.nodes" set are not making forward progress towards 
> decommissioning while the enqueued datanodes may be able to make forward 
> progress.
> h2. Reproduce Steps
>  * create a Hadoop cluster
>  * lose (i.e. terminate the host/process forever) over 100 datanodes while 
> the datanodes are in state decommissioning
>  * add additional datanodes to the cluster
>  * attempt to decommission those new datanodes & observe that they are stuck 
> in state decommissioning forever
> Note that in this example each datanode, over the full history of the 
> cluster, has a unique IP address



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