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https://issues.apache.org/jira/browse/HADOOP-692?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12467121
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dhruba borthakur commented on HADOOP-692:
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In the current code, topology information is NOT persisted into the fsimage.
This is perfect. However, when the namenode restarts it does not re-create the
topology map, it assumes that all datanodes are in a default-rack.
> Rack-aware Replica Placement
> ----------------------------
>
> Key: HADOOP-692
> URL: https://issues.apache.org/jira/browse/HADOOP-692
> Project: Hadoop
> Issue Type: Improvement
> Components: dfs
> Affects Versions: 0.8.0
> Reporter: Hairong Kuang
> Assigned To: Hairong Kuang
> Attachments: rack.patch, Rack_aware_HDFS_proposal.pdf
>
>
> This issue assumes that HDFS runs on a cluster of computers that spread
> across many racks. Communication between two nodes on different racks needs
> to go through switches. Bandwidth in/out of a rack may be less than the total
> bandwidth of machines in the rack. The purpose of rack-aware replica
> placement is to improve data reliability, availability, and network bandwidth
> utilization. The basic idea is that each data node determines to which rack
> it belongs at the startup time and notifies the name node of the rack id upon
> registration. The name node maintains a rackid-to-datanode map and tries to
> place replicas across racks.
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