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https://issues.apache.org/jira/browse/HDFS-12270?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Kihwal Lee reassigned HDFS-12270:
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Assignee: Kihwal Lee
> Allow more spreading of replicas during block placement
> -------------------------------------------------------
>
> Key: HDFS-12270
> URL: https://issues.apache.org/jira/browse/HDFS-12270
> Project: Hadoop HDFS
> Issue Type: Improvement
> Components: block placement
> Reporter: Kihwal Lee
> Assignee: Kihwal Lee
>
> The default block placement places the first replica locally if possible,
> then on a node in a remote rack, and finally another node in the remote rack.
> If more than 3 replicas are requested, the rest are spread across available
> racks. This strategy was chosen to minimize the inter-rack traffic and be
> able to tolerate a rack-level failure such as switch outages.
> This can tolerate a single rack failure, but if there also is a node outage
> (double failure), having missing blocks is highly likely. Although network
> bandwidth is still limited resource, it is less so than in the past. Some
> users might want increased data availability at the price of increased
> inter-rack traffic.
> This can be achieved by using the upgrade domain feature, but a simple tweak
> in the default policy can enable this, in case one does not want to go with
> the upgrade domain.
> I propose introducing a new config to control this.
> Rack placement level 0: default. Current behavior.
> Rack placement level 1: Use minimum 3 racks, if available. Allow existing
> blocks to remain as is.
> Rack placement level 2: Use minimum 3 racks, if available. Apply this policy
> to all replication verification. (e.g. replication queue initialization)
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