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From "Daryn Sharp (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-7964) Add support for async edit logging
Date Tue, 18 Aug 2015 19:21:47 GMT

    [ https://issues.apache.org/jira/browse/HDFS-7964?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14701823#comment-14701823
] 

Daryn Sharp commented on HDFS-7964:
-----------------------------------

[~hitliuyi], I'm sure I'll need to rebase.

The batched metric is the count of edits being implicitly sync'ed by another edit, not the
sum of all edits in the sync.  That was the pre-existing semantic I didn't want to alter.

Technically AsyncEdit#logSyncNotify doesn't need sync, but findbugs gets grumbly about access
to {{done}}.

Re: conf.  At the time, we didn't have much run time on it so I considered it experimental.
 All 2.6 clusters have been running with the feature enabled for 6+ months so I'll add to
the default conf.

> Add support for async edit logging
> ----------------------------------
>
>                 Key: HDFS-7964
>                 URL: https://issues.apache.org/jira/browse/HDFS-7964
>             Project: Hadoop HDFS
>          Issue Type: Sub-task
>          Components: namenode
>    Affects Versions: 2.0.2-alpha
>            Reporter: Daryn Sharp
>            Assignee: Daryn Sharp
>         Attachments: HDFS-7964.patch
>
>
> Edit logging is a major source of contention within the NN.  LogEdit is called within
the namespace write log, while logSync is called outside of the lock to allow greater concurrency.
 The handler thread remains busy until logSync returns to provide the client with a durability
guarantee for the response.
> Write heavy RPC load and/or slow IO causes handlers to stall in logSync.  Although the
write lock is not held, readers are limited/starved and the call queue fills.  Combining an
edit log thread with postponed RPC responses from HADOOP-10300 will provide the same durability
guarantee but immediately free up the handlers.



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