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42 lines
2.1 KiB
ReStructuredText
42 lines
2.1 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 atomically remove
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the metadata objects (pg->log_oid, pg->biginfo_oid) and rename the pg collections
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(temp, HEAD, and snap collections) into removal collections
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(see OSD::get_next_removal_coll). Those collections are then asynchronously
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removed. We do not do this inline because scanning the collections to remove
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the objects is an expensive operation. Atomically moving the directories out
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of the way allows us to proceed as if the pg is fully removed except that we
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cannot rewrite any of the objects contained in the removal directories until
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they have been fully removed. PGs partition the object space, so the only case
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we need to worry about is the same pg being recreated before we have finished
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removing the objects from the old one.
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OSDService::deleting_pgs tracks all pgs in the process of being deleted. Each
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DeletingState object in deleting_pgs lives while at least one reference to it
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remains. Each item in RemoveWQ carries a reference to the DeletingState for
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the relevant pg such that deleting_pgs.lookup(pgid) will return a null ref
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only if there are no collections currently being deleted for that pg.
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DeletingState allows you to register a callback to be called when the deletion
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is finally complete. See PG::start_flush. We use this mechanism to prevent
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the pg from being "flushed" until any pending deletes are complete. Metadata
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operations are safe since we did remove the old metadata objects and we
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inherit the osr from the previous copy of the pg.
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Similarly, OSD::osr_registry ensures that the OpSequencers for those pgs can
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be reused for a new pg if created before the old one is fully removed, ensuring
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that operations on the new pg are sequenced properly with respect to operations
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on the old one.
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OSD::load_pgs() rebuilds deleting_pgs and osr_registry when scanning the
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collections as it finds old removal collections not yet removed.
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