HDD I/O Explorer

▶ DRILL

Animated mechanical hard disk. Switch between SEQUENTIAL, RANDOM, and SCATTER/GATHER modes to see how seek time and rotational latency dominate I/O cost — and why they matter for SQL Server query plans.

slowfast

#Select a mode and press PLAY. SEQUENTIAL stays on one cylinder. RANDOM jumps tracks. SCATTER/GATHER shows DMA gather list execution.

# metrics

seeks0

rot. waits0

sectors read0

efficiency0%

⋯ rotational wait

■ reading

■ target sector

DMA buffer — 0 sectors assembled

# seek timeTime for the actuator arm to move to the target track — dominant cost for random I/O (~5–15 ms)

# rot. latencyWaiting for the target sector to spin under the head — avg ~4 ms at 7,200 RPM

# scatter/gatherDMA operation: one syscall assembles multiple non-contiguous disk regions into a single buffer without per-sector CPU involvement

# noteSimplified model — 6 tracks, 16 sectors, visual RPM. Real HDDs: hundreds of tracks, thousands of sectors, 7,200–15,000 RPM