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https://issues.apache.org/jira/browse/HDFS-528?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12739364#action_12739364
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dhruba borthakur commented on HDFS-528:
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Another generic approach is to specify the number of datanodes to wait for as
a percentage of the total number of datanodes in a cluster. You would have to
user the "includelist" feature of HDFS to list all the known datanodes (which
most admins probably do). In fact, the NN may exit safemode only if the
specified percentage of datanodes have checked in with the NN.
Many times, when we restart our cluster, many datanodes fail to join the NN.
However, the NN exists safemode because it finds at least one replica of every
block. Then the NN starts replicating blocks. We have to manually enter
safemode, manually look at the datanodes that have refuzed to join the NN, fix
them and then exit safemode. Your proposed feature helps in elegantly handling
this scenario.
> Add dfsadmin -waitDatanodes feature to block until DNs have reported
> --------------------------------------------------------------------
>
> Key: HDFS-528
> URL: https://issues.apache.org/jira/browse/HDFS-528
> Project: Hadoop HDFS
> Issue Type: New Feature
> Components: scripts
> Affects Versions: 0.21.0
> Reporter: Todd Lipcon
> Assignee: Todd Lipcon
> Attachments: hdfs-528.txt
>
>
> When starting up a fresh cluster programatically, users often want to wait
> until DFS is "writable" before continuing in a script. "dfsadmin -safemode
> wait" doesn't quite work for this on a completely fresh cluster, since when
> there are 0 blocks on the system, 100% of them are accounted for before any
> DNs have reported.
> This JIRA is to add a command which waits until a certain number of DNs have
> reported as alive to the NN.
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