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https://issues.apache.org/jira/browse/HDFS-528?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12742339#action_12742339
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Steve Loughran commented on HDFS-528:
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I do something like this in my workflow; block until the the #of workers is
above the required number. for one thing, that number could be >1, for another,
15, for a third 3000. It is also rought the same action regardless of whether
what you are waiting for is the NN or the JT; its the number of workers a
particular bit of the cluster has to help.
Rather than extend safemode, could you have something that just blocks off the
NN until its worker count is above what your specific script wants? After all,
it is your script that has needs, not the NN.
> Add ability for safemode to wait for a minimum number of live datanodes
> -----------------------------------------------------------------------
>
> 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-v2.txt, hdfs-528-v3.txt, 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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