Messenger throttle (number and size)
         |-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
                                                                     FileStore op_queue throttle (number and size)
                                                                |--------------------------------------------------------|
                                                                                                                                                                   WBThrottle
                                                                                                                       |---------------------------------------------------------------------------------------------------------|
                                                                                                                                                                 Journal (size)
                                                                                        |-----------------------------------------------------------------------------------------------------------------------------------------------------------------|
                                                                                                                       |----------------------------------------------------------------------------------------------------> flushed ----------------> synced
                                                                                                                       |
Op: Read Header --DispatchQ--> OSD::_dispatch --OpWQ--> PG::do_request --journalq--> Journal --FileStore::OpWQ--> Apply Thread --Finisher--> op_applied ------------------------------------------------------------->  Complete
                                              |                                                                                                                                                                      |
SubOp:                                        --Messenger--> ReadHeader --DispatchQ--> OSD::_dispatch --OpWQ--> PG::do_request --journalq--> Journal --FileStore::OpWQ--> Apply Thread --Finisher--> sub_op_applied -
                                                                                                                                                                              |
                                                                                                                                                                              |-----------------------------> flushed ----------------> synced
                                                                                                                                                |------------------------------------------------------------------------------------------|
                                                                                                                                                                                        Journal (size)
                                                                                                                                                                              |---------------------------------|
                                                                                                                                                                                        WBThrottle
                                                                                                                         |-----------------------------------------------------|
                                                                                                                            FileStore op_queue throttle (number and size)