In disk-performance terminology, what do we call the average time for the desired sector to rotate under the read/write head once the head is already positioned over the correct track?

Introduction / Context:
Overall disk access time is the sum of multiple components. Distinguishing these helps diagnose bottlenecks and compare storage technologies. After a head moves to the correct track, the disk must still rotate until the specific sector arrives under the head—this waiting period has its own name.

Given Data / Assumptions:

• Head is already on the right track (seek completed).
• We are waiting for the correct sector to come under the head.
• We want the standard performance term for that wait.

Concept / Approach:

Rotational delay (also called rotational latency) is the average time for one-half revolution at the drive’s rotational speed because, on average, the target sector is halfway around the disk when the head arrives at the track. Access time typically equals seek time + rotational delay + transfer time.

Step-by-Step Solution:

Verification / Alternative check:

Example: At 7,200 rpm, one rotation takes 8.33 ms, so average rotational delay ≈ 4.17 ms, which aligns with vendor specifications.

Why Other Options Are Wrong:

• Seek time: time to move heads to a track.
• Access time: combined metric, not the specific rotational component.
• Down time: period when equipment is unavailable due to failure or maintenance.
• None: incorrect because 'rotational delay' precisely matches.

Common Pitfalls:

• Using access time and rotational delay interchangeably; the former includes the latter.

Final Answer:

Rotational delay.