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https://issues.apache.org/jira/browse/HDFS-12128?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16085029#comment-16085029
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Brahma Reddy Battula commented on HDFS-12128:
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HDFS-11146 might help on this..? Even I considered this scenario.
> Namenode failover may make balancer's efforts be in vain
> --------------------------------------------------------
>
> Key: HDFS-12128
> URL: https://issues.apache.org/jira/browse/HDFS-12128
> Project: Hadoop HDFS
> Issue Type: Bug
> Components: balancer & mover
> Affects Versions: 2.6.0
> Reporter: liuyiyang
>
> The problem can be reproduced as follows:
> 1.In an HA cluster with imbalance datanode usage, we run "start-balancer.sh"
> to make the cluster balanced;
> 2.Before starting balancer, trigger failover of namenodes, this will make all
> datanodes be marked as stale by active namenode;
> 3.Start balancer to make the datanode usage balanced;
> 4.As balancer is running, under-utilized datanodes' usage will increase, but
> over-utilized datanodes' usage will stay unchanged for long time.
> Since all datanodes are marked as stale, deletion will be postponed in stale
> datanodes. During balancing, the replicas in source datanodes can't be
> deleted immediately,
> so the total usage of the cluster will increase and won't decrease until
> datanodes' stale state be cancelled.
> When the datanodes send next block report to namenode(default interval is
> 6h), active namenode will cancel the stale state of datanodes. I found if
> replicas on source datanodes can't be deleted immediately in OP_REPLACE
> operation via del_hint to namenode,
> namenode will schedule replicas on datanodes with least remaining space to
> delete instead of replicas on source datanodes. Unfortunately, datanodes with
> least remaining space may be the target datanodes when balancing, which will
> lead to imbalanced datanode usage again.
> If balancer finishes before next block report, all postponed over-replicated
> replicas will be deleted based on remaining space of datanodes, this may lead
> to furitless balancer efforts.
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