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diff --git a/doc/dev/peering.rst b/doc/dev/peering.rst new file mode 100644 index 00000000..7ee5debc --- /dev/null +++ b/doc/dev/peering.rst @@ -0,0 +1,259 @@ +====================== +Peering +====================== + +Concepts +-------- + +*Peering* + the process of bringing all of the OSDs that store + a Placement Group (PG) into agreement about the state + of all of the objects (and their metadata) in that PG. + Note that agreeing on the state does not mean that + they all have the latest contents. + +*Acting set* + the ordered list of OSDs who are (or were as of some epoch) + responsible for a particular PG. + +*Up set* + the ordered list of OSDs responsible for a particular PG for + a particular epoch according to CRUSH. Normally this + is the same as the *acting set*, except when the *acting set* has been + explicitly overridden via *PG temp* in the OSDMap. + +*PG temp* + a temporary placement group acting set used while backfilling the + primary osd. Let say acting is [0,1,2] and we are + active+clean. Something happens and acting is now [3,1,2]. osd 3 is + empty and can't serve reads although it is the primary. osd.3 will + see that and request a *PG temp* of [1,2,3] to the monitors using a + MOSDPGTemp message so that osd.1 temporarily becomes the + primary. It will select osd.3 as a backfill peer and continue to + serve reads and writes while osd.3 is backfilled. When backfilling + is complete, *PG temp* is discarded and the acting set changes back + to [3,1,2] and osd.3 becomes the primary. + +*current interval* or *past interval* + a sequence of OSD map epochs during which the *acting set* and *up + set* for particular PG do not change + +*primary* + the (by convention first) member of the *acting set*, + who is responsible for coordination peering, and is + the only OSD that will accept client initiated + writes to objects in a placement group. + +*replica* + a non-primary OSD in the *acting set* for a placement group + (and who has been recognized as such and *activated* by the primary). + +*stray* + an OSD who is not a member of the current *acting set*, but + has not yet been told that it can delete its copies of a + particular placement group. + +*recovery* + ensuring that copies of all of the objects in a PG + are on all of the OSDs in the *acting set*. Once + *peering* has been performed, the primary can start + accepting write operations, and *recovery* can proceed + in the background. + +*PG info* basic metadata about the PG's creation epoch, the version + for the most recent write to the PG, *last epoch started*, *last + epoch clean*, and the beginning of the *current interval*. Any + inter-OSD communication about PGs includes the *PG info*, such that + any OSD that knows a PG exists (or once existed) also has a lower + bound on *last epoch clean* or *last epoch started*. + +*PG log* + a list of recent updates made to objects in a PG. + Note that these logs can be truncated after all OSDs + in the *acting set* have acknowledged up to a certain + point. + +*missing set* + Each OSD notes update log entries and if they imply updates to + the contents of an object, adds that object to a list of needed + updates. This list is called the *missing set* for that <OSD,PG>. + +*Authoritative History* + a complete, and fully ordered set of operations that, if + performed, would bring an OSD's copy of a Placement Group + up to date. + +*epoch* + a (monotonically increasing) OSD map version number + +*last epoch start* + the last epoch at which all nodes in the *acting set* + for a particular placement group agreed on an + *authoritative history*. At this point, *peering* is + deemed to have been successful. + +*up_thru* + before a primary can successfully complete the *peering* process, + it must inform a monitor that is alive through the current + OSD map epoch by having the monitor set its *up_thru* in the osd + map. This helps peering ignore previous *acting sets* for which + peering never completed after certain sequences of failures, such as + the second interval below: + + - *acting set* = [A,B] + - *acting set* = [A] + - *acting set* = [] very shortly after (e.g., simultaneous failure, but staggered detection) + - *acting set* = [B] (B restarts, A does not) + +*last epoch clean* + the last epoch at which all nodes in the *acting set* + for a particular placement group were completely + up to date (both PG logs and object contents). + At this point, *recovery* is deemed to have been + completed. + +Description of the Peering Process +---------------------------------- + +The *Golden Rule* is that no write operation to any PG +is acknowledged to a client until it has been persisted +by all members of the *acting set* for that PG. This means +that if we can communicate with at least one member of +each *acting set* since the last successful *peering*, someone +will have a record of every (acknowledged) operation +since the last successful *peering*. +This means that it should be possible for the current +primary to construct and disseminate a new *authoritative history*. + +It is also important to appreciate the role of the OSD map +(list of all known OSDs and their states, as well as some +information about the placement groups) in the *peering* +process: + + When OSDs go up or down (or get added or removed) + this has the potential to affect the *active sets* + of many placement groups. + + Before a primary successfully completes the *peering* + process, the OSD map must reflect that the OSD was alive + and well as of the first epoch in the *current interval*. + + Changes can only be made after successful *peering*. + +Thus, a new primary can use the latest OSD map along with a recent +history of past maps to generate a set of *past intervals* to +determine which OSDs must be consulted before we can successfully +*peer*. The set of past intervals is bounded by *last epoch started*, +the most recent *past interval* for which we know *peering* completed. +The process by which an OSD discovers a PG exists in the first place is +by exchanging *PG info* messages, so the OSD always has some lower +bound on *last epoch started*. + +The high level process is for the current PG primary to: + + 1. get a recent OSD map (to identify the members of the all + interesting *acting sets*, and confirm that we are still the + primary). + + #. generate a list of *past intervals* since *last epoch started*. + Consider the subset of those for which *up_thru* was greater than + the first interval epoch by the last interval epoch's OSD map; that is, + the subset for which *peering* could have completed before the *acting + set* changed to another set of OSDs. + + Successful *peering* will require that we be able to contact at + least one OSD from each of *past interval*'s *acting set*. + + #. ask every node in that list for its *PG info*, which includes the most + recent write made to the PG, and a value for *last epoch started*. If + we learn about a *last epoch started* that is newer than our own, we can + prune older *past intervals* and reduce the peer OSDs we need to contact. + + #. if anyone else has (in its PG log) operations that I do not have, + instruct them to send me the missing log entries so that the primary's + *PG log* is up to date (includes the newest write).. + + #. for each member of the current *acting set*: + + a. ask it for copies of all PG log entries since *last epoch start* + so that I can verify that they agree with mine (or know what + objects I will be telling it to delete). + + If the cluster failed before an operation was persisted by all + members of the *acting set*, and the subsequent *peering* did not + remember that operation, and a node that did remember that + operation later rejoined, its logs would record a different + (divergent) history than the *authoritative history* that was + reconstructed in the *peering* after the failure. + + Since the *divergent* events were not recorded in other logs + from that *acting set*, they were not acknowledged to the client, + and there is no harm in discarding them (so that all OSDs agree + on the *authoritative history*). But, we will have to instruct + any OSD that stores data from a divergent update to delete the + affected (and now deemed to be apocryphal) objects. + + #. ask it for its *missing set* (object updates recorded + in its PG log, but for which it does not have the new data). + This is the list of objects that must be fully replicated + before we can accept writes. + + #. at this point, the primary's PG log contains an *authoritative history* of + the placement group, and the OSD now has sufficient + information to bring any other OSD in the *acting set* up to date. + + #. if the primary's *up_thru* value in the current OSD map is not greater than + or equal to the first epoch in the *current interval*, send a request to the + monitor to update it, and wait until receive an updated OSD map that reflects + the change. + + #. for each member of the current *acting set*: + + a. send them log updates to bring their PG logs into agreement with + my own (*authoritative history*) ... which may involve deciding + to delete divergent objects. + + #. await acknowledgment that they have persisted the PG log entries. + + #. at this point all OSDs in the *acting set* agree on all of the meta-data, + and would (in any future *peering*) return identical accounts of all + updates. + + a. start accepting client write operations (because we have unanimous + agreement on the state of the objects into which those updates are + being accepted). Note, however, that if a client tries to write to an + object it will be promoted to the front of the recovery queue, and the + write willy be applied after it is fully replicated to the current *acting set*. + + #. update the *last epoch started* value in our local *PG info*, and instruct + other *active set* OSDs to do the same. + + #. start pulling object data updates that other OSDs have, but I do not. We may + need to query OSDs from additional *past intervals* prior to *last epoch started* + (the last time *peering* completed) and following *last epoch clean* (the last epoch that + recovery completed) in order to find copies of all objects. + + #. start pushing object data updates to other OSDs that do not yet have them. + + We push these updates from the primary (rather than having the replicas + pull them) because this allows the primary to ensure that a replica has + the current contents before sending it an update write. It also makes + it possible for a single read (from the primary) to be used to write + the data to multiple replicas. If each replica did its own pulls, + the data might have to be read multiple times. + + #. once all replicas store the all copies of all objects (that + existed prior to the start of this epoch) we can update *last + epoch clean* in the *PG info*, and we can dismiss all of the + *stray* replicas, allowing them to delete their copies of objects + for which they are no longer in the *acting set*. + + We could not dismiss the *strays* prior to this because it was possible + that one of those *strays* might hold the sole surviving copy of an + old object (all of whose copies disappeared before they could be + replicated on members of the current *acting set*). + +State Model +----------- + +.. graphviz:: peering_graph.generated.dot |