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https://issues.apache.org/jira/browse/HDFS-17036?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17805895#comment-17805895
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ASF GitHub Bot commented on HDFS-17036:
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LiuGuH closed pull request #5710: HDFS-17036. Limit pendingReconstruction size
URL: https://github.com/apache/hadoop/pull/5710
> Limit pendingReconstruction size
> --------------------------------
>
> Key: HDFS-17036
> URL: https://issues.apache.org/jira/browse/HDFS-17036
> Project: Hadoop HDFS
> Issue Type: Improvement
> Components: hdfs
> Affects Versions: 3.4.0
> Reporter: liuguanghua
> Assignee: liuguanghua
> Priority: Major
> Labels: pull-request-available
>
> Consider the following scenario：
> (1) The hdfs cluster is big enough.
> Namenode Config：
> DFS_NAMENODE_REPLICATION_WORK_MULTIPLIER_PER_ITERATION ->200
> DFS_NAMENODE_REPLICATION_MAX_STREAMS_KEY->100
> DFS_NAMENODE_REPLICATION_STREAMS_HARD_LIMIT_KEY->200
> Datanode ：
> It has large disk on one datanode. One datanode has millions or even tens of
> millions blocks. Or maybe more in archive cluster.
> (2) The cluster administrator may do one of the following actions：
> 2.1 Increase replication on one large dir ,eg 2PB
> 2.2 Decommissioning some nodes from cluster
> (3) When the cluster datanode is on heavy read and write.
>
> Thus, datanode receive transfer commands form heartbeat whit namenode. When
> datanode is on heavily load （write and read）, the
> SumOfActorCommandQueueLength metris will increase to 1k+ because of slow
> consumption with lock competition. See details code ：
> BPServiceActor.CommandProcessingThread
>
> And Namenode distribute all replicationWork to datanode through heartbeat.
> All LowerReconstruction blocks will in neededReconstruction. Every heartbeat
> will distribute
> liveDatnaodes*DFS_NAMENODE_REPLICATION_WORK_MULTIPLIER_PER_ITERATION to the
> related datanodes. But as mentioned above，the datanode blocked executing the
> transfer block commands. When 5min later by default , the
> pendingReconstruction will into timeout and then re-entry
> neededReconstruction, and then re-entry pendingReconstruction . So the
> datanodes will receive multiple transfer command with same block. That will
> lead to excess blocks.
> Continuous cycle，it will lead to serious consequences. Datanode has more
> tranfers commands in queue and waste more disk space and result in more
> excess blocks.
>
> So, I think we should limit the max size of pendingReconstruction.
>
>
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