doc: add last_epoch_started.rst

Signed-off-by: Samuel Just <sjust@redhat.com>

diff --git a/doc/dev/osd_internals/last_epoch_started.rst b/doc/dev/osd_internals/last_epoch_started.rst
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+======================
+last_epoch_started
+======================
+
+info.last_epoch_started records an activation epoch e for interval i
+such that all writes commited 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
+have commited 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
+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
+(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
+upper bound on writes reported as committed to the client.
+
+We update info.last_epoch_started with the intial 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 designating a log entry divergent which in a prior interval
+would have been non-divergent.  In activate(), we use the peer's
+last_epoch_started value as a bound on how far back divergent log
+entries can be found.