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103 lines
4.0 KiB
ReStructuredText
103 lines
4.0 KiB
ReStructuredText
==========================
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Placement Group Concepts
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==========================
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When you execute commands like ``ceph -w``, ``ceph osd dump``, and other
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commands related to placement groups, Ceph may return values using some
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of the following terms:
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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 placement group.
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*Up Set*
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The ordered list of OSDs responsible for a particular placement
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group for 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
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been explicitly overridden via ``pg_temp`` in the OSD Map.
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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 placement group do not change.
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*Primary*
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The member (and by convention first) of the *Acting Set*,
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that 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 that 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 placement group
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are on all of the OSDs in the *Acting Set*. Once *Peering* has
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been performed, the *Primary* can start accepting write operations,
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and *Recovery* can proceed in the background.
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*PG Info*
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Basic metadata about the placement group's creation epoch, the version
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for the most recent write to the placement group, *last epoch started*,
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*last epoch clean*, and the beginning of the *current interval*. Any
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inter-OSD communication about placement groups includes the *PG Info*,
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such that any OSD that knows a placement group exists (or once existed)
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also has a lower 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 placement group.
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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 placement group 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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