mirror of
https://github.com/ceph/ceph
synced 2024-12-11 14:09:09 +00:00
6256d681a4
it is not easy to figure out what a PG temp is just by reading the code although it is easy to understand with an example. Signed-off-by: Loic Dachary <loic@dachary.org>
260 lines
11 KiB
ReStructuredText
260 lines
11 KiB
ReStructuredText
======================
|
|
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 with 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).
|
|
|
|
2. 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.
|
|
|
|
Successfull *peering* will require that we be able to contact at
|
|
least one OSD from each of *past interval*'s *acting set*.
|
|
|
|
3. 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.
|
|
|
|
5. if anyone else has (in his 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)..
|
|
|
|
5. for each member of the current *acting set*:
|
|
|
|
a) ask him 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 him 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, his 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.
|
|
|
|
b) ask him for his *missing set* (object updates recorded
|
|
in his PG log, but for which he does not have the new data).
|
|
This is the list of objects that must be fully replicated
|
|
before we can accept writes.
|
|
|
|
6. 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.
|
|
|
|
7. 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.
|
|
|
|
8. 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.
|
|
|
|
b) await acknowledgement that they have persisted the PG log entries.
|
|
|
|
9. 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*.
|
|
|
|
b) update the *last epoch started* value in our local *PG info*, and instruct
|
|
other *active set* OSDs to do the same.
|
|
|
|
c) 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.
|
|
|
|
d) 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.
|
|
|
|
10. 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
|