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From "Andrew Wang (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-5064) Standby checkpoints should not block concurrent readers
Date Fri, 07 Mar 2014 23:24:45 GMT

    [ https://issues.apache.org/jira/browse/HDFS-5064?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13924477#comment-13924477
] 

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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