X-Git-Url: https://gerrit.opnfv.org/gerrit/gitweb?a=blobdiff_plain;f=src%2Fceph%2Fdoc%2Fdev%2Fosd_internals%2Flast_epoch_started.rst;fp=src%2Fceph%2Fdoc%2Fdev%2Fosd_internals%2Flast_epoch_started.rst;h=9978bd31c1fda342dc5f6b2e894d7952a6534b14;hb=812ff6ca9fcd3e629e49d4328905f33eee8ca3f5;hp=0000000000000000000000000000000000000000;hpb=15280273faafb77777eab341909a3f495cf248d9;p=stor4nfv.git

diff --git a/src/ceph/doc/dev/osd_internals/last_epoch_started.rst b/src/ceph/doc/dev/osd_internals/last_epoch_started.rst
new file mode 100644
index 0000000..9978bd3
--- /dev/null
+++ b/src/ceph/doc/dev/osd_internals/last_epoch_started.rst
@@ -0,0 +1,60 @@
+======================
+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 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
+how far back divergent log entries can be found.
+
+However, in a case like
+
+.. code::
+
+  calc_acting osd.0 1.4e( v 473'302 (292'200,473'302] local-les=473 n=4 ec=5 les/c 473/473 556/556/556
+  calc_acting osd.1 1.4e( v 473'302 (293'202,473'302] lb 0//0//-1 local-les=477 n=0 ec=5 les/c 473/473 556/556/556
+  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
+cannot have served reads.