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https://issues.apache.org/jira/browse/HDFS-5064?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13924477#comment-13924477
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Andrew Wang commented on HDFS-5064:
-----------------------------------
Restating my +1, going to commit this shortly.
> Standby checkpoints should not block concurrent readers
> -------------------------------------------------------
>
> Key: HDFS-5064
> URL: https://issues.apache.org/jira/browse/HDFS-5064
> Project: Hadoop HDFS
> Issue Type: Bug
> Components: ha, namenode
> Affects Versions: 2.3.0
> Reporter: Aaron T. Myers
> Assignee: Aaron T. Myers
> Attachments: HDFS-5064.patch, HDFS-5064.patch
>
>
> We've observed an issue which causes fetches of the {{/jmx}} page of the NN
> to take a long time to load when the standby is in the process of creating a
> checkpoint.
> Even though both creating the checkpoint and gathering the statistics for
> {{/jmx}} take only the FSNS read lock, the issue is that since the FSNS uses
> a _fair_ RW lock, a single writer attempting to get the lock will block all
> threads attempting to get only the read lock for the duration of the
> checkpoint. This will cause {{/jmx}}, and really any thread only attempting
> to get the read lock, to block for the duration of the checkpoint, even
> though they should be able to proceed concurrently with the checkpointing
> thread.
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