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75 lines
2.8 KiB
ReStructuredText
75 lines
2.8 KiB
ReStructuredText
====================
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Recovery Reservation
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====================
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Recovery reservation extends and subsumes backfill reservation. The
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reservation system from backfill recovery is used for local and remote
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reservations.
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When a PG goes active, first it determines what type of recovery is
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necessary, if any. It may need log-based recovery, backfill recovery,
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both, or neither.
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In log-based recovery, the primary first acquires a local reservation
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from the OSDService's local_reserver. Then a MRemoteReservationRequest
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message is sent to each replica in order of OSD number. These requests
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will always be granted (i.e., cannot be rejected), but they may take
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some time to be granted if the remotes have already granted all their
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remote reservation slots.
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After all reservations are acquired, log-based recovery proceeds as it
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would without the reservation system.
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After log-based recovery completes, the primary releases all remote
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reservations. The local reservation remains held. The primary then
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determines whether backfill is necessary. If it is not necessary, the
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primary releases its local reservation and waits in the Recovered state
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for all OSDs to indicate that they are clean.
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If backfill recovery occurs after log-based recovery, the local
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reservation does not need to be reacquired since it is still held from
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before. If it occurs immediately after activation (log-based recovery
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not possible/necessary), the local reservation is acquired according to
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the typical process.
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Once the primary has its local reservation, it requests a remote
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reservation from the backfill target. This reservation CAN be rejected,
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for instance if the OSD is too full (osd_backfill_full_ratio config
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option). If the reservation is rejected, the primary drops its local
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reservation, waits (osd_backfill_retry_interval), and then retries. It
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will retry indefinitely.
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Once the primary has the local and remote reservations, backfill
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proceeds as usual. After backfill completes the remote reservation is
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dropped.
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Finally, after backfill (or log-based recovery if backfill was not
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necessary), the primary drops the local reservation and enters the
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Recovered state. Once all the PGs have reported they are clean, the
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primary enters the Clean state and marks itself active+clean.
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--------------
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Things to Note
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--------------
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We always grab the local reservation first, to prevent a circular
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dependency. We grab remote reservations in order of OSD number for the
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same reason.
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The recovery reservation state chart controls the PG state as reported
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to the monitor. The state chart can set:
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- recovery_wait: waiting for local/remote reservations
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- recovering: recoverying
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- wait_backfill: waiting for remote backfill reservations
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- backfilling: backfilling
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- backfill_toofull: backfill reservation rejected, OSD too full
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--------
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See Also
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--------
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The Active substate of the automatically generated OSD state diagram.
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