c/partition_allocator: maintain and use final partition counts #11154
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When moving partitions, partition allocations on old nodes are not removed until the move is finished. This makes sense because partitions remain physically present on old nodes until the very end. But we also use these allocation counts for count-based rebalancing. This is incorrect because after all moves are finished, the distribution won't necessary be uniform. To fix this, introduce "final counts" - the number of partitions on a node after all currently in-progress moves are finished and maintain it when (re)allocating partitions and in topic_updates_dispatcher.
Choose a node for the replica based on final partition counts, not on currently allocated partitions (this should give better results for counts-based rebalancing). Additionally, rename these constraints to better reflect what they are doing.
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mmaslankaprv
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Jun 2, 2023
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When moving partitions, partition allocations on old nodes are not removed until the move is finished. This makes sense because partitions remain physically present on old nodes until the very end. But we also use these allocation counts for count-based rebalancing. This is incorrect because after all moves are finished, the distribution won't necessary be uniform.
To fix this, introduce "final counts" - the number of partitions on a node after all currently in-progress moves are finished, maintain it when (re)allocating partitions and in
topic_updates_dispatcherand use it in placement constraints and partition counts rebalancing inmembers_backend.Backports Required
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