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diff --git a/doc/dev/osd_internals/backfill_reservation.rst b/doc/dev/osd_internals/backfill_reservation.rst new file mode 100644 index 00000000..cf9dab4d --- /dev/null +++ b/doc/dev/osd_internals/backfill_reservation.rst @@ -0,0 +1,38 @@ +==================== +Backfill Reservation +==================== + +When a new osd joins a cluster, all pgs containing it must eventually backfill +to it. If all of these backfills happen simultaneously, it would put excessive +load on the osd. osd_max_backfills limits the number of outgoing or +incoming backfills on a single node. The maximum number of outgoing backfills is +osd_max_backfills. The maximum number of incoming backfills is +osd_max_backfills. Therefore there can be a maximum of osd_max_backfills * 2 +simultaneous backfills on one osd. + +Each OSDService now has two AsyncReserver instances: one for backfills going +from the osd (local_reserver) and one for backfills going to the osd +(remote_reserver). An AsyncReserver (common/AsyncReserver.h) manages a queue +by priority of waiting items and a set of current reservation holders. When a +slot frees up, the AsyncReserver queues the Context* associated with the next +item on the highest priority queue in the finisher provided to the constructor. + +For a primary to initiate a backfill, it must first obtain a reservation from +its own local_reserver. Then, it must obtain a reservation from the backfill +target's remote_reserver via a MBackfillReserve message. This process is +managed by substates of Active and ReplicaActive (see the substates of Active +in PG.h). The reservations are dropped either on the Backfilled event, which +is sent on the primary before calling recovery_complete and on the replica on +receipt of the BackfillComplete progress message), or upon leaving Active or +ReplicaActive. + +It's important that we always grab the local reservation before the remote +reservation in order to prevent a circular dependency. + +We want to minimize the risk of data loss by prioritizing the order in +which PGs are recovered. The highest priority is log based recovery +(OSD_RECOVERY_PRIORITY_MAX) since this must always complete before +backfill can start. The next priority is backfill of degraded PGs and +is a function of the degradation. A backfill for a PG missing two +replicas will have a priority higher than a backfill for a PG missing +one replica. The lowest priority is backfill of non-degraded PGs. |