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Date: Mon, 23 Jul 2012 11:03:34 +0000 (UTC)
From: "nkeywal (JIRA)" <jira@apache.org>
To: hdfs-issues@hadoop.apache.org
Message-ID: <541368796.90543.1343041414929.JavaMail.jiratomcat@issues-vm>
Subject: [jira] [Created] (HDFS-3703) Decrease the datanode failure
 detection time
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nkeywal created HDFS-3703:
-----------------------------

             Summary: 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: data-node, name-node
    Affects Versions: 2.0.0-alpha, 1.0.3
            Reporter: nkeywal


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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