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 groups.
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.