StoreWritePressure

The append latency of one store, as the node's logs on it measure each call of append and appendAll (the backend's own time, the wait for a connection included; not the partition append queue's): a sample is the 90th percentile of the period's latencies against a baseline of the earlier periods.

The baseline is the median of the latest BASELINE_PERIODS periods' percentiles: the store's normal latency, which neither its quietest seconds pull down nor a burst of slow ones pulls up. A period of fewer than MIN_APPENDS appends says nothing of the latency and leaves the baseline as it is. A period keeps at most the latest MAX_SAMPLES latencies.

Like any baseline that follows its store, a pressure that lasts longer than half the window becomes the store's normal latency; the callers waiting for a connection, which the pool says, press all the same.