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https://issues.apache.org/jira/browse/HDFS-11586?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16752986#comment-16752986
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Zheng Shao commented on HDFS-11586:
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[~vagarychen] I would propose that "%free + %write_locked + %read_locked =
100%". This probably answers your question about what is defined as "%free".
I am thinking about collecting 1 data point per 10ms, and then report the 3
metrics above every 1 second (or 100 data points x 10ms each) via JMX metrics.
> Report %free, %write_locked, %read_locked for the NameNode FSNamesystemLock
> ---------------------------------------------------------------------------
>
> Key: HDFS-11586
> URL: https://issues.apache.org/jira/browse/HDFS-11586
> Project: Hadoop HDFS
> Issue Type: Improvement
> Components: namenode
> Reporter: Zheng Shao
> Assignee: Chen Liang
> Priority: Minor
>
> It's useful to understand how busy the NameNode is by providing these
> metrics, similar to the %util number from iostat for disks.
> When %free goes to close to 0, we know the NameNode is congested (just like
> when disk %util goes to 100%).
> This can be implemented very cheaply by using a thread that wakes up every
> 10ms to check FSNamesystemLock's getReadLockCount() and isWriteLocked() (via
> the member coarseLock).
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