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https://issues.apache.org/jira/browse/HADOOP-3002?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12611387#action_12611387
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dhruba borthakur commented on HADOOP-3002:
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Hi Konstantin, I opened HADOOP-3709 to document the lock-hierarchy violation in 
processing heartbeats. The goal is to not acquire the global FSNamesystem lock 
to process every heartbeat. Maybe the patch you provide in this patch already 
fixes HADOOP-3709.

> HDFS should not remove blocks while in safemode.
> ------------------------------------------------
>
>                 Key: HADOOP-3002
>                 URL: https://issues.apache.org/jira/browse/HADOOP-3002
>             Project: Hadoop Core
>          Issue Type: Bug
>          Components: dfs
>    Affects Versions: 0.16.0
>            Reporter: Konstantin Shvachko
>            Assignee: Konstantin Shvachko
>            Priority: Blocker
>             Fix For: 0.17.2, 0.18.0
>
>         Attachments: DelBlocksInSafeMode-018.patch, DelBlocksInSafeMode.patch
>
>
> I noticed that data-nodes are removing blocks during a rather prolonged 
> distributed upgrade when the name-node is in safe mode.
> This happened on my experimental cluster with accelerated block report rate.
> By definition in safe mode the name-node should not
> - accept client requests to change the namespace state, and
> - schedule block replications and/or block removal for the data-nodes.
> We don't want any unnecessary replications until all blocks are reported 
> during startup.
> We also don't want to remove blocks if safe mode is entered manually.
> In heartbeat processing we explicitly verify that the name-node is in 
> safe-mode and do not return any block commands to the data-nodes.
> Block reports can also return block commands, which should be banned during 
> safe mode.

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