[HDFS-9044] Give Priority to FavouredNodes , before selecting nodes from FavouredNode's Node Group

• Type: Bug
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Fix Version/s: 2.8.0, 3.0.0-alpha1
• Hadoop Flags: Reviewed
• Assignee: J.Andreina
• Reporter: J.Andreina
• Created: 10/Sep/15 11:33
• Updated: 30/Aug/16 01:27
• Resolved: 29/Oct/15 06:17

Description

Passing Favored nodes intention is to place replica among the favored node

Current behavior in Node group is
If favored node is not available it goes to one among favored nodegroup.

Say for example:
	1)I need 3 replicas and passed 5 favored nodes.
	2)Out of 5 favored nodes 3 favored nodes are not good.
	3)Then based on BlockPlacementPolicyWithNodeGroup out of 5 targets node returned , 3 will be random node from 3 bad FavoredNode's nodegroup. 
	4)Then there is a probability that all my 3 replicas are placed on Random node from FavoredNodes's nodegroup , instead of giving priority to 2 favored nodes returned as target.

Instead of returning 5 targets on 3rd step above , we can return 2 good favored nodes as target
And remaining 1 needed replica can be chosen from Random node of bad FavoredNodes's nodegroup.
This will make sure that the FavoredNodes are given priority.