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From "Ming Ma (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-10206) getBlockLocations might not sort datanodes properly by distance
Date Mon, 28 Nov 2016 20:08:58 GMT

    [ https://issues.apache.org/jira/browse/HDFS-10206?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15702984#comment-15702984
] 

Ming Ma commented on HDFS-10206:
--------------------------------

* NetworkTopology can be used by HDFS, YARN and MAPREDUCE. It is better to make variable names
more general.

bq. Out of three replica, one will be in off rack datanode which is causing the difference
But the reader should pick the closest one, either "Same Node" and "DataNode in same rack".
Perhaps you can clarify the setup.

bq. Weight calculation after this patch
So the weight value definition has changed. It should be fine given it isn't a public interface.
Still NetworkTopologyWithNodeGroup has its own getWeight definition based on the old definition.
Either we update that or keep the weight value.



> getBlockLocations might not sort datanodes properly by distance
> ---------------------------------------------------------------
>
>                 Key: HDFS-10206
>                 URL: https://issues.apache.org/jira/browse/HDFS-10206
>             Project: Hadoop HDFS
>          Issue Type: Improvement
>            Reporter: Ming Ma
>            Assignee: Nandakumar
>         Attachments: HDFS-10206.000.patch, HDFS-10206.001.patch, HDFS-10206.002.patch
>
>
> If the DFSClient machine is not a datanode, but it shares its rack with some datanodes
of the HDFS block requested, {{DatanodeManager#sortLocatedBlocks}} might not put the local-rack
datanodes at the beginning of the sorted list. That is because the function didn't call {{networktopology.add(client);}}
to properly set the node's parent node; something required by {{networktopology.sortByDistance}}
to compute distance between two nodes in the same topology tree.
> Another issue with {{networktopology.sortByDistance}} is it only distinguishes local
rack from remote rack, but it doesn't support general distance calculation to tell how remote
the rack is.
> {noformat}
> NetworkTopology.java
>   protected int getWeight(Node reader, Node node) {
>     // 0 is local, 1 is same rack, 2 is off rack
>     // Start off by initializing to off rack
>     int weight = 2;
>     if (reader != null) {
>       if (reader.equals(node)) {
>         weight = 0;
>       } else if (isOnSameRack(reader, node)) {
>         weight = 1;
>       }
>     }
>     return weight;
>   }
> {noformat}
> HDFS-10203 has suggested moving the sorting from namenode to DFSClient to address another
issue. Regardless of where we do the sorting, we still need fix the issues outline here.
> Note that BlockPlacementPolicyDefault shares the same NetworkTopology object used by
DatanodeManager and requires Nodes stored in the topology to be {{DatanodeDescriptor}} for
block placement. So we need to make sure we don't pollute the  NetworkTopology if we plan
to fix it on the server side.



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