[HDFS-7097] Allow block reports to be processed during checkpointing on standby name node

• Type: Bug
• Status: Closed
• Priority: Critical
• Resolution: Fixed
• Fix Version/s: 2.7.0
• Assignee: Kihwal Lee
• Reporter: Kihwal Lee
• Created: 19/Sep/14 14:47
• Updated: 18/Nov/16 19:17
• Resolved: 26/Nov/14 16:56

Description

On a reasonably busy HDFS cluster, there are stream of creates, causing data nodes to generate incremental block reports. When a standby name node is checkpointing, RPC handler threads trying to process a full or incremental block report is blocked on the name system's fsLock, because the checkpointer acquires the read lock on it. This can create a serious problem if the size of name space is big and checkpointing takes a long time.

All available RPC handlers can be tied up very quickly. If you have 100 handlers, it only takes 34 file creates. If a separate service RPC port is not used, HA transition will have to wait in the call queue for minutes. Even if a separate service RPC port is configured, hearbeats from datanodes will be blocked. A standby NN with a big name space can lose all data nodes after checkpointing. The rpc calls will also be retransmitted by data nodes many times, filling up the call queue and potentially causing listen queue overflow.

Since block reports are not modifying any state that is being saved to fsimage, I propose letting them through during checkpointing.

Issue Links

• breaks: HDFS-7448 TestBookKeeperHACheckpoints fails in trunk build
• duplicates: HDFS-10214 Checkpoint Can not be done by StandbyNameNode.Because checkpoint may cause DataNode blockReport.blockReceivedAndDeleted.heartbeat rpc timeout when the object num > 100000000.
• relates to: HDFS-11157 Enhance documentation around cpLock