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https://issues.apache.org/jira/browse/HADOOP-4584?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12671244#action_12671244
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Suresh Srinivas commented on HADOOP-4584:
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1. {{dataAvailable.wait (time till next report);}}
because currently the loop exit happens when {{shouldRun}} is to {{false}} or
{{shutdown()}} is called. Assuming that "time till next report" is time till
next block report, the {{offerService()}} does not end for a long time. This
delays shutdown in some of the cases and results in few unit testcase failures
that assumes datanode shuts down quickly. Alternatively we could notify
{{dataAvailable}} when {{shouldRun}} is set to false. But I think that makes
the code quite ugly. Hence the 1 second {{wait}} time.
2. Using dataAvailable for synchronizing commandQ and receivedBlockList
I think currently dataAvailable is used to wake up a thread that is waiting for
either a command or a received block. Data can be added to commandQ or
receivedBlock independent of each other. Doing this assumes that a command
cannot added while received block is being added to receivedBlockList.
> Slow generation of blockReport at DataNode causes delay of sending heartbeat
> to NameNode
> ----------------------------------------------------------------------------------------
>
> Key: HADOOP-4584
> URL: https://issues.apache.org/jira/browse/HADOOP-4584
> Project: Hadoop Core
> Issue Type: Bug
> Components: dfs
> Reporter: Hairong Kuang
> Assignee: Suresh Srinivas
> Fix For: 0.20.0
>
> Attachments: 4584.patch, 4584.patch, 4584.patch
>
>
> sometimes due to disk or some other problems, datanode takes minutes or tens
> of minutes to generate a block report. It causes the datanode not able to
> send heartbeat to NameNode every 3 seconds. In the worst case, it makes
> NameNode to detect a lost heartbeat and wrongly decide that the datanode is
> dead.
> It would be nice to have two threads instead. One thread is for scanning data
> directories and generating block report, and executes the requests sent by
> NameNode; Another thread is for sending heartbeats, block reports, and
> picking up the requests from NameNode. By having these two threads, the
> sending of heartbeats will not get delayed by any slow block report or slow
> execution of NameNode requests.
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