[HADOOP-16161] NetworkTopology#getWeightUsingNetworkLocation return unexpected result

• Type: Bug
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Fix Version/s: 3.3.0, 3.1.4, 3.2.2
• Component/s: net
• Hadoop Flags: Reviewed
• Assignee: Xiaoqiao He
• Reporter: Xiaoqiao He
• Created: 04/Mar/19 13:41
• Updated: 23/Mar/22 04:30
• Resolved: 13/May/19 18:46

Description

Consider the following scenario:
1. there are 4 slaves and topology like:
Rack: /IDC/RACK1
hostname1
hostname2
Rack: /IDC/RACK2
hostname3
hostname4
2. Reader from hostname1, and calculate weight between reader and [hostname1, hostname3, hostname4] by #getWeight, and their corresponding values are [0,4,4]
3. Reader from client which is not in the topology, and in the same IDC but in none rack of the topology, and calculate weight between reader and [hostname1, hostname3, hostname4] by #getWeightUsingNetworkLocation, and their corresponding values are [2,2,2]
4. Other different Reader can get the similar results.
The weight result for case #3 is obviously not the expected value, the truth is [4,4,4]. this issue may cause reader not really following arrange: local -> local rack -> remote rack.

After dig the detailed implement, the root cause is #getWeightUsingNetworkLocation only calculate distance between Racks rather than hosts.

I think we should add constant 2 to correct the weight of #getWeightUsingNetworkLocation.

Issue Links

• relates to: HDFS-16517 In 2.10 the distance metric is wrong for non-DN machines