[HDFS-5064] Standby checkpoints should not block concurrent readers

• Type: Bug
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Affects Version/s: 2.3.0
• Fix Version/s: 2.4.0
• Component/s: ha, namenode
• Target Version/s: 2.1.1-beta
• Assignee: Aaron Myers
• Reporter: Aaron Myers
• Created: 03/Aug/13 00:24
• Updated: 15/Apr/14 00:11
• Resolved: 07/Mar/14 23:41

Description

We've observed an issue which causes fetches of the /jmx page of the NN to take a long time to load when the standby is in the process of creating a checkpoint.

Even though both creating the checkpoint and gathering the statistics for /jmx take only the FSNS read lock, the issue is that since the FSNS uses a fair RW lock, a single writer attempting to get the lock will block all threads attempting to get only the read lock for the duration of the checkpoint. This will cause /jmx, and really any thread only attempting to get the read lock, to block for the duration of the checkpoint, even though they should be able to proceed concurrently with the checkpointing thread.