This is a follow up of the comment on HADOOP-12916 and here add more metrics and WARN/DEBUG logs for long FSD/FSN locking operations on namenode similar to what we have for slow write/network WARN/metrics on datanode.
Throttle FsNameSystem lock warnings
Tracking methods that hold FSNamesystemLock for too long
Measure lock time in FsDatasetImpl
Add Logging for Long-held NN Read Locks
SampleQuantile does not perform well under load