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+==========================
+ Placement Group Concepts
+==========================
+
+When you execute commands like ``ceph -w``, ``ceph osd dump``, and other 
+commands related to placement groups, Ceph may return values using some
+of the following terms: 
+
+*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 placement group.
+
+*Up Set*
+   The ordered list of OSDs responsible for a particular placement
+   group 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 OSD Map.
+
+*Current Interval* or *Past Interval*
+   A sequence of OSD map epochs during which the *Acting Set* and *Up
+   Set* for particular placement group do not change.
+
+*Primary*
+   The member (and by convention first) of the *Acting Set*,
+   that 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 that 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 placement group
+   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 placement group's creation epoch, the version
+   for the most recent write to the placement group, *last epoch started*, 
+   *last epoch clean*, and the beginning of the *current interval*.  Any
+   inter-OSD communication about placement groups includes the *PG Info*, 
+   such that any OSD that knows a placement group 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 placement group.
+   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 placement group logs and object contents).
+   At this point, *recovery* is deemed to have been
+   completed.