PastIntervals
Purpose
There are two situations where we need to consider the set of all acting-set
OSDs for a PG back to some epoch e:
During peering, we need to consider the acting set for every epoch back to
last_epoch_started, the last epoch in which the PG completed peering and became active. (see last_epoch_started for a detailed explanation)During recovery, we need to consider the acting set for every epoch back to
last_epoch_clean, the last epoch at which all of the OSDs in the acting set were fully recovered, and the acting set was full.
For either of these purposes, we could build such a set by iterating backwards from the current OSDMap to the relevant epoch. Instead, we maintain a structure PastIntervals for each PG.
An interval is a contiguous sequence of OSDMap epochs where the PG mapping
didn’t change. This includes changes to the acting set, the up set, the
primary, and several other parameters fully spelled out in
PastIntervals::check_new_interval.
Maintenance and Trimming
The PastIntervals structure stores a record for each interval back to
last_epoch_clean. On each new interval (See AdvMap reactions,
PeeringState::should_restart_peering, and PeeringState::start_peering_interval)
each OSD with the PG will add the new interval to its local PastIntervals.
Activation messages to OSDs which do not already have the PG contain the
sender’s PastIntervals so that the recipient needn’t rebuild it. (See
PeeringState::activate needs_past_intervals).
PastIntervals are trimmed in two places. First, when the primary marks the PG clean, it clears its past_intervals instance (PeeringState::try_mark_clean()). The replicas will do the same thing when they receive the info (See PeeringState::update_history).
The second, more complex, case is in PeeringState::start_peering_interval. In
the event of a “map gap”, we assume that the PG actually has gone clean, but we
haven’t received a pg_info_t with the updated last_epoch_clean value yet.
To explain this behavior, we need to discuss OSDMap trimming.
OSDMap Trimming
OSDMaps are created by the Monitor quorum and gossiped out to the OSDs. The Monitor cluster also determines when OSDs (and the Monitors) are allowed to trim old OSDMap epochs. For the reasons explained above in this document, the primary constraint is that we must retain all OSDMaps back to some epoch such that all PGs have been clean at that or a later epoch (min_last_epoch_clean). (See OSDMonitor::get_trim_to).
The Monitor quorum determines min_last_epoch_clean through MOSDBeacon messages sent periodically by each OSDs. Each message contains a set of PGs for which the OSD is primary at that moment as well as the min_last_epoch_clean across that set. The Monitors track these values in OSDMonitor::last_epoch_clean.
There is a subtlety in the min_last_epoch_clean value used by the OSD to
populate the MOSDBeacon. OSD::collect_pg_stats invokes PG::with_pg_stats to
obtain the lec value, which actually uses
pg_stat_t::get_effective_last_epoch_clean() rather than
info.history.last_epoch_clean. If the PG is currently clean,
pg_stat_t::get_effective_last_epoch_clean() is the current epoch rather than
last_epoch_clean – this works because the PG is clean at that epoch and it
allows OSDMaps to be trimmed during periods where OSDMaps are being created
(due to snapshot activity, perhaps), but no PGs are undergoing interval
changes.
Back to PastIntervals
We can now understand our second trimming case above. If OSDMaps have been
trimmed up to epoch e, we know that the PG must have been clean at some epoch
>= e (indeed, all PGs must have been), so we can drop our PastIntevals.
This dependency also pops up in PeeringState::check_past_interval_bounds().
PeeringState::get_required_past_interval_bounds takes as a parameter
oldest_epoch, which comes from OSDSuperblock::cluster_osdmap_trim_lower_bound.
We use cluster_osdmap_trim_lower_bound rather than a specific osd’s oldest_map
because we don’t necessarily trim all MOSDMap::cluster_osdmap_trim_lower_bound.
In order to avoid doing too much work at once we limit the amount of osdmaps
trimmed using osd_target_transaction_size in OSD::trim_maps().
For this reason, a specific OSD’s oldest_map can lag behind
OSDSuperblock::cluster_osdmap_trim_lower_bound
for a while.
See https://tracker.ceph.com/issues/49689 for an example.