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Date: Tue, 24 Feb 2009 12:05:02 -0800 (PST)
From: "Raghu Angadi (JIRA)" <jira@apache.org>
To: core-dev@hadoop.apache.org
Subject: [jira] Commented: (HADOOP-4584) Slow generation of blockReport at
 DataNode causes delay of sending heartbeat to NameNode
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    [ https://issues.apache.org/jira/browse/HADOOP-4584?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12676396#action_12676396 ] 

Raghu Angadi commented on HADOOP-4584:
--------------------------------------


Just moving heartbeat to a different thread is useful by itself. My quote from 4th comment above :

bq. "I think this approach might work ok for now. It makes sure the data node is not marked dead. But this should be considered mostly a work around. We should note the fundamental problem still remains (a little less lethal). e.g. a) new blocks are not reported, b) no new blocks can be written during this time c) (not sure) not blocks can be read? etc. "

If that is what everyone wants then it is ok. But we will continue to see problems with datanodes with lot of blocks. My impression is that it is about time we fix the fundamental problem with the scan in block reports.

> 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, 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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