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From "Nikola Vujic (JIRA)" <j...@apache.org>
Subject [jira] [Created] (HADOOP-9936) BlockPlacementPolicy does not work for cross rack/node group dependencies
Date Thu, 05 Sep 2013 19:45:53 GMT
Nikola Vujic created HADOOP-9936:

             Summary: BlockPlacementPolicy does not work for cross rack/node group dependencies
                 Key: HADOOP-9936
                 URL: https://issues.apache.org/jira/browse/HADOOP-9936
             Project: Hadoop Common
          Issue Type: Improvement
            Reporter: Nikola Vujic
            Priority: Critical

Block placement policies do not work for cross rack/node group dependencies. In reality this
is needed when compute servers and storage fall in two independent fault domains, then both
BlockPlacementPolicyDefault and BlockPlacementPolicyWithNodeGroup are not able to provide
proper block placement.

Let's suppose that we have Hadoop cluster with one rack with two servers, and we run 2 VMs
per server. Node group topology for this cluster would be:
 server1-vm1 -> /d1/r1/n1
 server1-vm2 -> /d1/r1/n1
 server2-vm1 -> /d1/r1/n2
 server2-vm2 -> /d1/r1/n2

This is working fine as long as server and storage fall into the same fault domain but if
storage is in a different fault domain from the server, we will not be able to handle that.
For example, if storage of server1-vm1 is in the same fault domain as storage of server2-vm1,
then we must not place two replicas on these two nodes although they are in different node

Two possible approaches:
- One approach would be to define cross rack/node group dependencies and to use them when
excluding nodes from the search space. This looks as the cleanest way to fix this as it requires
minor changes in the BlockPlacementPolicy classes.
- Other approach would be to allow nodes to fall in more than one node group. When we chose
a node to hold a replica we have to exclude from the search space all nodes from the node
groups where the chosen node belongs. This approach may require major changes in the NetworkTopology.

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