[ https://issues.apache.org/jira/browse/HDFS-10206?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Nandakumar updated HDFS-10206:
------------------------------
Attachment: HDFS-10206.003.patch
> 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,
HDFS-10206.003.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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