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28 lines
1.4 KiB
ReStructuredText
28 lines
1.4 KiB
ReStructuredText
====================
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Backfill Reservation
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====================
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When a new osd joins a cluster, all pgs containing it must eventually backfill
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to it. If all of these backfills happen simultaneously, it would put excessive
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load on the osd. osd_num_concurrent_backfills limits the number of outgoing or
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incoming backfills on a single node.
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Each OSDService now has two AsyncReserver instances: one for backfills going
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from the osd (local_reserver) and one for backfills going to the osd
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(remote_reserver). An AsyncReserver (common/AsyncReserver.h) manages a queue
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of waiting items and a set of current reservation holders. When a slot frees
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up, the AsyncReserver queues the Context* associated with the next item in the
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finisher provided to the constructor.
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For a primary to initiate a backfill, it must first obtain a reservation from
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its own local_reserver. Then, it must obtain a reservation from the backfill
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target's remote_reserver via a MBackfillReserve message. This process is
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managed by substates of Active and ReplicaActive (see the substates of Active
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in PG.h). The reservations are dropped either on the Backfilled event, which
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is sent on the primary before calling recovery_complete and on the replica on
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receipt of the BackfillComplete progress message), or upon leaving Active or
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ReplicaActive.
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It's important that we always grab the local reservation before the remote
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reservation in order to prevent a circular dependency.
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