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https://issues.apache.org/jira/browse/HDFS-550?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12753203#action_12753203
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Hairong Kuang commented on HDFS-550:
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> This is harmless because they will get deleted in the next block report,
> isn't it?
No, this is not harmless. That's the key point. Block report does not handle
duplicate replicas from one datanode. More problematic is that the temporary
replicas under "detached" may be corrupt if DataNode dies when copy is in
progress. So recovery introduces corrupt replicas.
> DataNode restarts may introduce corrupt/duplicated/lost replicas when
> handling detached replicas
> ------------------------------------------------------------------------------------------------
>
> Key: HDFS-550
> URL: https://issues.apache.org/jira/browse/HDFS-550
> Project: Hadoop HDFS
> Issue Type: Sub-task
> Components: data-node
> Affects Versions: 0.21.0
> Reporter: Hairong Kuang
> Assignee: Hairong Kuang
> Priority: Blocker
> Fix For: Append Branch
>
> Attachments: detach.patch
>
>
> Current trunk first calls detach to unlinks a finalized replica before
> appending to this block. Unlink is done by temporally copying the block file
> in the "current" subtree to a directory called "detach" under the volume's
> daa directory and then copies it back when unlink succeeds. On datanode
> restarts, datanodes recover faied unlink by copying replicas under "detach"
> to "current".
> There are two bugs with this implementation:
> 1. The "detach" directory does not include in a snapshot. so rollback will
> cause the "detaching" replicas to be lost.
> 2. After a replica is copied to the "detach" directory, the information of
> its original location is lost. The current implementation erroneously assumes
> that the replica to be unlinked is under "current". This will make two
> instances of replicas with the same block id to coexist in a datanode. Also
> if a replica under "detach" is corrupt, the corrupt replica is moved to
> "current" without being detected, polluting datanode data.
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