mirror of https://github.com/ceph/ceph
61 lines
3.2 KiB
ReStructuredText
61 lines
3.2 KiB
ReStructuredText
======================
|
|
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/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 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
|
|
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 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
|
|
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.
|