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57 lines
2.5 KiB
ReStructuredText
57 lines
2.5 KiB
ReStructuredText
==========
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PG Removal
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==========
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See OSD::_remove_pg, OSD::RemoveWQ
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There are two ways for a pg to be removed from an OSD:
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1. MOSDPGRemove from the primary
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2. OSD::advance_map finds that the pool has been removed
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In either case, our general strategy for removing the pg is to
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atomically set the metadata objects (pg->log_oid, pg->biginfo_oid) to
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backfill and asynronously remove the pg collections. We do not do
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this inline because scanning the collections to remove the objects is
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an expensive operation.
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OSDService::deleting_pgs tracks all pgs in the process of being
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deleted. Each DeletingState object in deleting_pgs lives while at
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least one reference to it remains. Each item in RemoveWQ carries a
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reference to the DeletingState for the relevant pg such that
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deleting_pgs.lookup(pgid) will return a null ref only if there are no
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collections currently being deleted for that pg.
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The DeletingState for a pg also carries information about the status
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of the current deletion and allows the deletion to be cancelled.
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The possible states are:
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1. QUEUED: the PG is in the RemoveWQ
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2. CLEARING_DIR: the PG's contents are being removed synchronously
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3. DELETING_DIR: the PG's directories and metadata being queued for removal
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4. DELETED_DIR: the final removal transaction has been queued
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5. CANCELED: the deletion has been canceled
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In 1 and 2, the deletion can be canceled. Each state transition
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method (and check_canceled) returns false if deletion has been
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canceled and true if the state transition was successful. Similarly,
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try_stop_deletion() returns true if it succeeds in canceling the
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deletion. Additionally, try_stop_deletion() in the event that it
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fails to stop the deletion will not return until the final removal
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transaction is queued. This ensures that any operations queued after
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that point will be ordered after the pg deletion.
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OSD::_create_lock_pg must handle two cases:
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1. Either there is no DeletingStateRef for the pg, or it failed to cancel
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2. We succeeded in canceling the deletion.
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In case 1., we proceed as if there were no deletion occurring, except that
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we avoid writing to the PG until the deletion finishes. In case 2., we
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proceed as in case 1., except that we first mark the PG as backfilling.
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Similarly, OSD::osr_registry ensures that the OpSequencers for those
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pgs can be reused for a new pg if created before the old one is fully
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removed, ensuring that operations on the new pg are sequenced properly
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with respect to operations on the old one.
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