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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 Sat, 03 Dec 2016 04:29:58 GMT

    [ https://issues.apache.org/jira/browse/HDFS-10206?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15717370#comment-15717370
] 

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

Thanks [~nandakumar131]! The patches look good overall. To make the method more general, seems
better to have getWeightUsingNetworkLocation return 0 when two nodes have the same network
path. [~daryn] [~kihwal], any concerns about the added 0.1ms latency? Note this only happens
for non-datanode reader scenario and it doesn't hold FSNamesystem lock.

> 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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