[HDFS-4093] In branch-1-win, AzureBlockPlacementPolicy#chooseTarget only returns one DN when replication factor is greater than 3.

• Type: Bug
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Fix Version/s: 1-win
• Hadoop Flags: Reviewed
• Assignee: Jing Zhao
• Reporter: Jing Zhao
• Created: 19/Oct/12 22:23
• Updated: 24/Oct/12 00:02
• Resolved: 24/Oct/12 00:02

Description

In branch-1-win, when AzureBlockPlacementPolicy (which extends the BlockPlacementPolicyDefault) is used, if the client increases the number of replicas (e.g., from 3 to 10), AzureBlockPlacementPolicy#chooseTarget will return only 1 Datanode each time. Thus in FSNameSystem#computeReplicationWorkForBlock, it is possible that the replication monitor may choose a datanode that has been chosen as target but still in the pendingReplications (because computeReplicationWorkForBlock does not check the pending replication before doing the chooseTarget).

To avoid this "hit-the-same-datanode" scenario, we modify the AzureBlockPlacementPolicy#chooseTarget to make it return multiple DN.