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From "dhruba borthakur (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HDFS-550) DataNode restarts may introduce corrupt/duplicated/lost replicas when handling detached replicas
Date Wed, 09 Sep 2009 19:46:57 GMT

    [ https://issues.apache.org/jira/browse/HDFS-550?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12753233#action_12753233
] 

dhruba borthakur commented on HDFS-550:
---------------------------------------

The recovery code (at datanode restart time) first checks to see if the block is already in
current. If not, it adds it to current. So, there can be only replica of that block in the
"current" directory of the namenode.

Another copy of the same block could also reside in the "snapshot" directory. Is that what
you mean by "duplicate replicas on the same datanode"?

> 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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