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