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https://issues.apache.org/jira/browse/HDFS-1972?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13168624#comment-13168624
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dhruba borthakur commented on HDFS-1972:
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findOverReplicatedReplicas runs with the readLock. chooseExcessReplicates runs
without any lock but returns it results rather than modifying any global data
structures. Then we acquire the fsnamesystem lock, quickly validate that the
state of the block has not changed since we did all the computation (and most
of the time the state does not change), and then proceed to do the action.
> HA: Datanode fencing mechanism
> ------------------------------
>
> Key: HDFS-1972
> URL: https://issues.apache.org/jira/browse/HDFS-1972
> Project: Hadoop HDFS
> Issue Type: Sub-task
> Components: data-node, ha, name-node
> Reporter: Suresh Srinivas
> Assignee: Todd Lipcon
> Attachments: hdfs-1972-v1.txt, hdfs-1972.txt
>
>
> In high availability setup, with an active and standby namenode, there is a
> possibility of two namenodes sending commands to the datanode. The datanode
> must honor commands from only the active namenode and reject the commands
> from standby, to prevent corruption. This invariant must be complied with
> during fail over and other states such as split brain. This jira addresses
> issues related to this, design of the solution and implementation.
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