[ https://issues.apache.org/jira/browse/HADOOP-692?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12463431 ]
Hairong Kuang commented on HADOOP-692: -------------------------------------- Although the 2nd replica is placed off-rack, the 3rd replica is placed on the same rack as the first replica. Besides chooseTarget returns a sorted list of the choosen targets so that they form a shortest path that starts from the writer and traverses all the targets. So in the returned list, the 1st element is the local node, 2nd is the same-rack node, and 3rd is the off-rack node. > 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. -- This message is automatically generated by JIRA. - If you think it was sent incorrectly contact one of the administrators: https://issues.apache.org/jira/secure/Administrators.jspa - For more information on JIRA, see: http://www.atlassian.com/software/jira