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https://issues.apache.org/jira/browse/HDFS-11960?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16055768#comment-16055768
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Daryn Sharp commented on HDFS-11960:
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+1 Looks good. It's great to have finally solved this vexing problem with
persistent under-replication!
> Successfully closed files can stay under-replicated.
> ----------------------------------------------------
>
> Key: HDFS-11960
> URL: https://issues.apache.org/jira/browse/HDFS-11960
> Project: Hadoop HDFS
> Issue Type: Bug
> Reporter: Kihwal Lee
> Assignee: Kihwal Lee
> Priority: Critical
> Attachments: HDFS-11960.patch, HDFS-11960-v2.branch-2.txt,
> HDFS-11960-v2.trunk.txt
>
>
> If a certain set of conditions hold at the time of a file creation, a block
> of the file can stay under-replicated. This is because the block is
> mistakenly taken out of the under-replicated block queue and never gets
> reevaluated.
> Re-evaluation can be triggered if
> - a replica containing node dies.
> - setrep is called
> - NN repl queues are reinitialized (NN failover or restart)
> If none of these happens, the block stays under-replicated.
> Here is how it happens.
> 1) A replica is finalized, but the ACK does not reach the upstream in time.
> IBR is also delayed.
> 2) A close recovery happens, which updates the gen stamp of "healthy"
> replicas.
> 3) The file is closed with the healthy replicas. It is added to the
> replication queue.
> 4) A replication is scheduled, so it is added to the pending replication
> list. The replication target is picked as the failed node in 1).
> 5) The old IBR is finally received for the failed/excluded node. In the
> meantime, the replication fails, because there is already a finalized replica
> (with older gen stamp) on the node.
> 6) The IBR processing removes the block from the pending list, adds it to
> corrupt replicas list, and then issues invalidation. Since the block is in
> neither replication queue nor pending list, it stays under-replicated.
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