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https://issues.apache.org/jira/browse/HADOOP-4584?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12689604#action_12689604
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dhruba borthakur commented on HADOOP-4584:
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Nice document. Thanks.
When the DirectoryScanner scans the directory, does it need to keep the
FSDataSet lock? If not, can you pl explain (in the document) why? What makes
the DirectoryScanner scan all the blocks in the data directory atomically?
> 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.brthread.2.patch, 4584.brthread.3.patch,
> 4584.brthread.3.patch, 4584.brthread.3.patch, 4584.brthread.3.patch,
> 4584.brthread.3.patch, 4584.hbthread.patch, 4584.patch, 4584.patch,
> 4584.patch, 4584.patch, 4584.patch, 4584.patch, Design.pdf
>
>
> 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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