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Diffstat (limited to 'src/ceph/doc/dev/osd_internals/last_epoch_started.rst')
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diff --git a/src/ceph/doc/dev/osd_internals/last_epoch_started.rst b/src/ceph/doc/dev/osd_internals/last_epoch_started.rst deleted file mode 100644 index 9978bd3..0000000 --- a/src/ceph/doc/dev/osd_internals/last_epoch_started.rst +++ /dev/null @@ -1,60 +0,0 @@ -====================== -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. |