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https://issues.apache.org/jira/browse/HDFS-550?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12748056#action_12748056
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Konstantin Shvachko commented on HDFS-550:
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Suffix ".unlinked" would be better.
> 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
>
>
> 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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