[HDFS-9945] Datanode command for evicting writers

• Type: New Feature
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Fix Version/s: 2.8.0, 3.0.0-alpha1
• Component/s: datanode
• Target Version/s: 2.8.0
• Hadoop Flags: Reviewed

Release Note

This new dfsadmin command, evictWriters, stops active block writing activities on a data node. The affected writes will continue without the node after a write pipeline recovery. This is useful when data node decommissioning is blocked by slow writers. If issued against a non-decommissioing data node, all current writers will be stopped, but new write requests will continue to be served.

• Assignee: Kihwal Lee
• Reporter: Kihwal Lee
• Created: 11/Mar/16 14:36
• Updated: 30/Aug/16 01:18
• Resolved: 07/Apr/16 12:47

Description

It will be useful if there is a command to evict writers from a datanode. When a set of datanodes are being decommissioned, they can get blocked by slow writers at the end. It was rare in the old days since mapred jobs didn't last too long, but with many different types of apps running on today's YARN cluster, we are often see very long tail in datanode decommissioning.

I propose a new dfsadmin command, evictWriters, to be added. I initially thought about having namenode automatically telling datanodes on decommissioning, but realized that having a command is more flexible. E.g. users can choose not to do this at all, choose when to evict writers, or whether to try multiple times for whatever reasons.

Issue Links

• is related to: HDFS-10280 Document new dfsadmin command -evictWriters