mirror of https://github.com/ceph/ceph
39 lines
2.1 KiB
ReStructuredText
39 lines
2.1 KiB
ReStructuredText
====================
|
|
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.
|