--- /dev/null
+===========================
+Map and PG Message handling
+===========================
+
+Overview
+--------
+The OSD handles routing incoming messages to PGs, creating the PG if necessary
+in some cases.
+
+PG messages generally come in two varieties:
+
+ 1. Peering Messages
+ 2. Ops/SubOps
+
+There are several ways in which a message might be dropped or delayed. It is
+important that the message delaying does not result in a violation of certain
+message ordering requirements on the way to the relevant PG handling logic:
+
+ 1. Ops referring to the same object must not be reordered.
+ 2. Peering messages must not be reordered.
+ 3. Subops must not be reordered.
+
+MOSDMap
+-------
+MOSDMap messages may come from either monitors or other OSDs. Upon receipt, the
+OSD must perform several tasks:
+
+ 1. Persist the new maps to the filestore.
+ Several PG operations rely on having access to maps dating back to the last
+ time the PG was clean.
+ 2. Update and persist the superblock.
+ 3. Update OSD state related to the current map.
+ 4. Expose new maps to PG processes via *OSDService*.
+ 5. Remove PGs due to pool removal.
+ 6. Queue dummy events to trigger PG map catchup.
+
+Each PG asynchronously catches up to the currently published map during
+process_peering_events before processing the event. As a result, different
+PGs may have different views as to the "current" map.
+
+One consequence of this design is that messages containing submessages from
+multiple PGs (MOSDPGInfo, MOSDPGQuery, MOSDPGNotify) must tag each submessage
+with the PG's epoch as well as tagging the message as a whole with the OSD's
+current published epoch.
+
+MOSDPGOp/MOSDPGSubOp
+--------------------
+See OSD::dispatch_op, OSD::handle_op, OSD::handle_sub_op
+
+MOSDPGOps are used by clients to initiate rados operations. MOSDSubOps are used
+between OSDs to coordinate most non peering activities including replicating
+MOSDPGOp operations.
+
+OSD::require_same_or_newer map checks that the current OSDMap is at least
+as new as the map epoch indicated on the message. If not, the message is
+queued in OSD::waiting_for_osdmap via OSD::wait_for_new_map. Note, this
+cannot violate the above conditions since any two messages will be queued
+in order of receipt and if a message is received with epoch e0, a later message
+from the same source must be at epoch at least e0. Note that two PGs from
+the same OSD count for these purposes as different sources for single PG
+messages. That is, messages from different PGs may be reordered.
+
+
+MOSDPGOps follow the following process:
+
+ 1. OSD::handle_op: validates permissions and crush mapping.
+ discard the request if they are not connected and the client cannot get the reply ( See OSD::op_is_discardable )
+ See OSDService::handle_misdirected_op
+ See PG::op_has_sufficient_caps
+ See OSD::require_same_or_newer_map
+ 2. OSD::enqueue_op
+
+MOSDSubOps follow the following process:
+
+ 1. OSD::handle_sub_op checks that sender is an OSD
+ 2. OSD::enqueue_op
+
+OSD::enqueue_op calls PG::queue_op which checks waiting_for_map before calling OpWQ::queue which adds the op to the queue of the PG responsible for handling it.
+
+OSD::dequeue_op is then eventually called, with a lock on the PG. At
+this time, the op is passed to PG::do_request, which checks that:
+
+ 1. the PG map is new enough (PG::must_delay_op)
+ 2. the client requesting the op has enough permissions (PG::op_has_sufficient_caps)
+ 3. the op is not to be discarded (PG::can_discard_{request,op,subop,scan,backfill})
+ 4. the PG is active (PG::flushed boolean)
+ 5. the op is a CEPH_MSG_OSD_OP and the PG is in PG_STATE_ACTIVE state and not in PG_STATE_REPLAY
+
+If these conditions are not met, the op is either discarded or queued for later processing. If all conditions are met, the op is processed according to its type:
+
+ 1. CEPH_MSG_OSD_OP is handled by PG::do_op
+ 2. MSG_OSD_SUBOP is handled by PG::do_sub_op
+ 3. MSG_OSD_SUBOPREPLY is handled by PG::do_sub_op_reply
+ 4. MSG_OSD_PG_SCAN is handled by PG::do_scan
+ 5. MSG_OSD_PG_BACKFILL is handled by PG::do_backfill
+
+CEPH_MSG_OSD_OP processing
+--------------------------
+
+PrimaryLogPG::do_op handles CEPH_MSG_OSD_OP op and will queue it
+
+ 1. in wait_for_all_missing if it is a CEPH_OSD_OP_PGLS for a designated snapid and some object updates are still missing
+ 2. in waiting_for_active if the op may write but the scrubber is working
+ 3. in waiting_for_missing_object if the op requires an object or a snapdir or a specific snap that is still missing
+ 4. in waiting_for_degraded_object if the op may write an object or a snapdir that is degraded, or if another object blocks it ("blocked_by")
+ 5. in waiting_for_backfill_pos if the op requires an object that will be available after the backfill is complete
+ 6. in waiting_for_ack if an ack from another OSD is expected
+ 7. in waiting_for_ondisk if the op is waiting for a write to complete
+
+Peering Messages
+----------------
+See OSD::handle_pg_(notify|info|log|query)
+
+Peering messages are tagged with two epochs:
+
+ 1. epoch_sent: map epoch at which the message was sent
+ 2. query_epoch: map epoch at which the message triggering the message was sent
+
+These are the same in cases where there was no triggering message. We discard
+a peering message if the message's query_epoch if the PG in question has entered
+a new epoch (See PG::old_peering_evt, PG::queue_peering_event). Notifies,
+infos, notifies, and logs are all handled as PG::RecoveryMachine events and
+are wrapped by PG::queue_* by PG::CephPeeringEvts, which include the created
+state machine event along with epoch_sent and query_epoch in order to
+generically check PG::old_peering_message upon insertion and removal from the
+queue.
+
+Note, notifies, logs, and infos can trigger the creation of a PG. See
+OSD::get_or_create_pg.
+
+
Code Review / stor4nfv.git / blobdiff