A simple read interface provides uniform semantics but unpredictable performance. Using synchronous large-buffer Linux reads, we decompose the buffered-read path into work volume, processing time, and critical-path exposure. Reducing local cost or accelerating request submission need not shorten a complete read. Linux uses large folios to reduce metadata work and I/O-copy overlap to hide most cold-read copying; an experimental kernel prototype shows that more aggressive overlap and parallelism can shorten the host path when these mechanisms fail. Device waiting dominates SSD cold reads. We abstract first-completion wait F and subsequent completion capacity B_CQ from request-batch timelines. Real-SSD experiments characterize their variation with request and batch size, while MQSim traces explain their internal origins. A wait chain along the folio-bio-request mapping connects device completion to upper-layer waiting. Under strict batching, the median absolute cross-layer prediction error is at most 7.43%. Finally, we apply the completion model and wait chain to request supply, Linux readahead, data layout, and waiting policies. Cases cover gains, no gains, and reversals, comparing predictions with interventions to close the loop from observation and modeling to understanding and control and provide a measurable basis for layered I/O decisions.

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