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Chris LuandGitHub
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* topology: track volume size only where writes can land Size tracking decays pending assignment estimates so the master does not overfill a volume before heartbeats catch up. Nothing is ever assigned to a read-only volume, so an entry for one can never be consulted -- and in a tiered cluster that is most of them, which made this the volume layout's largest cost. A volume held out of the writable list for capacity is not read-only and keeps its entry: that entry is what enforces the recovery delay. 800k volumes, 90% read-only: sizeTracking 79.1MB -> 8.4MB, and the crowded set falls out with it because a read-only volume no longer reaches the threshold check at all. * topology: decide size tracking per volume, not per reporting replica A volume is unwritable if any replica is read-only, so asking the replica whose heartbeat happened to arrive made the answer depend on arrival order: a writable replica reporting after a read-only one put the tracking back. Ask the volume instead, which also drops the caller-supplied flag and the churn it caused. The crowded entry goes with the tracking, since leaving it behind would only move the memory this releases. Costs a map lookup per replica on a full-list heartbeat, about 19ms per 100k volumes and no allocations, on a path that is now rare.