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Date: Thu, 12 Apr 2012 05:58:35 +0000 (UTC)
From: "Aaron T. Myers (Commented) (JIRA)" <jira@apache.org>
To: hdfs-issues@hadoop.apache.org
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Subject: [jira] [Commented] (HDFS-3256) HDFS considers blocks
 under-replicated if topology script is configured with only 1 rack
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    [ https://issues.apache.org/jira/browse/HDFS-3256?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13252227#comment-13252227 ] 

Aaron T. Myers commented on HDFS-3256:
--------------------------------------

bq. How does this relate to AbstractDNSToSwitchMapping.isSingleSwitch that Steve added recently?

I think the two are unrelated. This change only determines rack cardinality using the NetworkTopology class, which itself is fed by the configured DNSToSwitchMapping. This is the same methodology that the BlockManager has thus far been using to determine if replicas for a given block exist on more than one rack or not.
                
> HDFS considers blocks under-replicated if topology script is configured with only 1 rack
> ----------------------------------------------------------------------------------------
>
>                 Key: HDFS-3256
>                 URL: https://issues.apache.org/jira/browse/HDFS-3256
>             Project: Hadoop HDFS
>          Issue Type: Bug
>    Affects Versions: 2.0.0
>            Reporter: Aaron T. Myers
>            Assignee: Aaron T. Myers
>         Attachments: HDFS-3256.patch, HDFS-3256.patch
>
>
> HDFS treats the mere presence of a topology script being configured as evidence that there are multiple racks. If there is in fact only a single rack, the NN will try to place the blocks on at least two racks, and thus blocks will be considered to be under-replicated.

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