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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
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Aaron T. Myers commented on HDFS-3256:
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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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