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https://issues.apache.org/jira/browse/HDFS-5168?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13762553#comment-13762553
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Junping Du commented on HDFS-5168:
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IMO, this is a more generic scope effort as proposing storage group as an
orthogonal failure group that across previous network location. I would
recommend you to start from a proposal for everyone to discuss. Also,
delivering a demo patch would help others understanding better. Thoughts?
Thanks again for working on this. Nikola!
> BlockPlacementPolicy does not work for cross rack/node group dependencies
> -------------------------------------------------------------------------
>
> Key: HDFS-5168
> URL: https://issues.apache.org/jira/browse/HDFS-5168
> Project: Hadoop HDFS
> Issue Type: Improvement
> Reporter: Nikola Vujic
> Assignee: 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 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.
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