[jira] [Created] (HDFS-5064) Standby checkpoints should not block concurrent readers

["Aaron T. Myers (JIRA)" <ji...@apache.org>]

Aaron T. Myers created HDFS-5064:
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             Summary: 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: 2.1.1-beta
            Reporter: Aaron T. Myers
            Assignee: Aaron T. Myers


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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