last_epoch_started
======================
-info.last_epoch_started records an activation epoch e for interval i
-such that all writes committed in i or earlier are reflected in the
-local info/log and no writes after i are reflected in the local
+``info.last_epoch_started`` records an activation epoch ``e`` for interval ``i``
+such that all writes committed in ``i`` or earlier are reflected in the
+local info/log and no writes after ``i`` are reflected in the local
info/log. Since no committed write is ever divergent, even if we
-get an authoritative log/info with an older info.last_epoch_started,
-we can leave our info.last_epoch_started alone since no writes could
+get an authoritative log/info with an older ``info.last_epoch_started``,
+we can leave our ``info.last_epoch_started`` alone since no writes could
have committed in any intervening interval (See PG::proc_master_log).
-info.history.last_epoch_started records a lower bound on the most
-recent interval in which the pg as a whole went active and accepted
-writes. On a particular osd, it is also an upper bound on the
-activation epoch of intervals in which writes in the local pg log
-occurred (we update it before accepting writes). Because all
-committed writes are committed by all acting set osds, any
-non-divergent writes ensure that history.last_epoch_started was
+``info.history.last_epoch_started`` records a lower bound on the most
+recent interval in which the PG as a whole went active and accepted
+writes. On a particular OSD it is also an upper bound on the
+activation epoch of intervals in which writes in the local PG log
+occurred: we update it before accepting writes. Because all
+committed writes are committed by all acting set OSDs, any
+non-divergent writes ensure that ``history.last_epoch_started`` was
recorded by all acting set members in the interval. Once peering has
-queried one osd from each interval back to some seen
-history.last_epoch_started, it follows that no interval after the max
-history.last_epoch_started can have reported writes as committed
+queried one OSD from each interval back to some seen
+``history.last_epoch_started``, it follows that no interval after the max
+``history.last_epoch_started`` can have reported writes as committed
(since we record it before recording client writes in an interval).
-Thus, the minimum last_update across all infos with
-info.last_epoch_started >= MAX(history.last_epoch_started) must be an
+Thus, the minimum ``last_update`` across all infos with
+``info.last_epoch_started >= MAX(history.last_epoch_started)`` must be an
upper bound on writes reported as committed to the client.
-We update info.last_epoch_started with the initial activation message,
-but we only update history.last_epoch_started after the new
-info.last_epoch_started is persisted (possibly along with the first
-write). This ensures that we do not require an osd with the most
-recent info.last_epoch_started until all acting set osds have recorded
+We update ``info.last_epoch_started`` with the initial activation message,
+but we only update ``history.last_epoch_started`` after the new
+``info.last_epoch_started`` is persisted (possibly along with the first
+write). This ensures that we do not require an OSD with the most
+recent ``info.last_epoch_started`` until all acting set OSDs have recorded
it.
-In find_best_info, we do include info.last_epoch_started values when
-calculating the max_last_epoch_started_found because we want to avoid
+In ``find_best_info``, we do include ``info.last_epoch_started`` values when
+calculating ``max_last_epoch_started_found`` because we want to avoid
designating a log entry divergent which in a prior interval would have
been non-divergent since it might have been used to serve a read. In
-activate(), we use the peer's last_epoch_started value as a bound on
+``activate()``, we use the peer's ``last_epoch_started`` value as a bound on
how far back divergent log entries can be found.
However, in a case like
calc_acting osd.4 1.4e( v 473'302 (120'121,473'302] local-les=473 n=4 ec=5 les/c 473/473 556/556/556
calc_acting osd.5 1.4e( empty local-les=0 n=0 ec=5 les/c 473/473 556/556/556
-since osd.1 is the only one which recorded info.les=477 while 4,0
-which were the acting set in that interval did not (4 restarted and 0
-did not get the message in time) the pg is marked incomplete when
-either 4 or 0 would have been valid choices. To avoid this, we do not
-consider info.les for incomplete peers when calculating
-min_last_epoch_started_found. It would not have been in the acting
-set, so we must have another osd from that interval anyway (if
-maybe_went_rw). If that osd does not remember that info.les, then we
+since osd.1 is the only one which recorded info.les=477, while osd.4,osd.0
+(which were the acting set in that interval) did not (osd.4 restarted and osd.0
+did not get the message in time), the PG is marked incomplete when
+either osd.4 or osd.0 would have been valid choices. To avoid this, we do not
+consider ``info.les`` for incomplete peers when calculating
+``min_last_epoch_started_found``. It would not have been in the acting
+set, so we must have another OSD from that interval anyway (if
+``maybe_went_rw``). If that OSD does not remember that ``info.les``, then we
cannot have served reads.
projects / ceph.git / blobdiff