+++ /dev/null
-==========================
- 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.
Code Review / stor4nfv.git / blobdiff