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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