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From "Varun Sharma (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-3703) Decrease the datanode failure detection time
Date Fri, 19 Apr 2013 23:15:16 GMT

    [ https://issues.apache.org/jira/browse/HDFS-3703?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13636998#comment-13636998
] 

Varun Sharma commented on HDFS-3703:
------------------------------------

Do you know if for a block which is not FINALIZED and is UNDER_CONSTRUCTION state, whether
namenode returns all 3 datanodes or it just returns the 1st datanode in the pipeline (the
block on datanode side in the RBW, replica being written state) ?
                
> Decrease the datanode failure detection time
> --------------------------------------------
>
>                 Key: HDFS-3703
>                 URL: https://issues.apache.org/jira/browse/HDFS-3703
>             Project: Hadoop HDFS
>          Issue Type: Improvement
>          Components: datanode, namenode
>    Affects Versions: 1.0.3, 2.0.0-alpha, 3.0.0
>            Reporter: Nicolas Liochon
>            Assignee: Jing Zhao
>             Fix For: 1.1.0, 2.0.3-alpha
>
>         Attachments: 3703-hadoop-1.0.txt, HDFS-3703-branch-1.1-read-only.patch, HDFS-3703-branch-1.1-read-only.patch,
HDFS-3703-branch2.patch, HDFS-3703.patch, HDFS-3703-trunk-read-only.patch, HDFS-3703-trunk-read-only.patch,
HDFS-3703-trunk-read-only.patch, HDFS-3703-trunk-read-only.patch, HDFS-3703-trunk-read-only.patch,
HDFS-3703-trunk-read-only.patch, HDFS-3703-trunk-read-only.patch, HDFS-3703-trunk-with-write.patch
>
>
> By default, if a box dies, the datanode will be marked as dead by the namenode after
10:30 minutes. In the meantime, this datanode will still be proposed  by the nanenode to write
blocks or to read replicas. It happens as well if the datanode crashes: there is no shutdown
hooks to tell the nanemode we're not there anymore.
> It especially an issue with HBase. HBase regionserver timeout for production is often
30s. So with these configs, when a box dies HBase starts to recover after 30s and, while 10
minutes, the namenode will consider the blocks on the same box as available. Beyond the write
errors, this will trigger a lot of missed reads:
> - during the recovery, HBase needs to read the blocks used on the dead box (the ones
in the 'HBase Write-Ahead-Log')
> - after the recovery, reading these data blocks (the 'HBase region') will fail 33% of
the time with the default number of replica, slowering the data access, especially when the
errors are socket timeout (i.e. around 60s most of the time). 
> Globally, it would be ideal if HDFS settings could be under HBase settings. 
> As a side note, HBase relies on ZooKeeper to detect regionservers issues.

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