Performance Lens

• For added clones/allocations on request/object/block paths, quantify copied data and frequency. Recommend borrowing, move, Bytes/Arc, Cow, or capacity reservation only for a concrete repeated cost.
• Route every new sync/flush through the durability-mode and bucket override gates; mode none must not pay the new fsync.
• Keep blocking filesystem/CPU work off async runtime threads, but do not split one small operation into many spawn_blocking round trips.
• Measure lock hold time across I/O and compare acquisition order for ABBA.
• Keep cleanup, extra stat/rename, and diagnostics out of the PUT commit critical section when they need not be there.
• Detect per-item serial I/O/RPC in batch APIs and accidental quadratic scans; use a gate or bounded concurrency when the concrete fan-out warrants it.
• Count buffer growth and byte copies in EC/bitrot paths; preserve pool gauge balance and avoid repeated metadata decode/fetch per object.
• Repetitive success logs stay at trace; metrics/instrumentation on hot paths require an existing gate.
• Claims of no impact on PUT/GET/commit/erasure paths need relevant benchmark or A/B evidence, especially for 4 KiB objects.