On magnetic disks, what is the term for the set of tracks that lie directly above one another on all platters and can be accessed without moving the read/write heads (i.e., without a seek operation)?

Introduction / Context:
Understanding physical disk geometry helps explain why some I/O operations are faster than others. By minimizing head movement (seeks), the system can read related blocks with lower latency. The construct that groups same-radius tracks across all platters is central to that optimization.

Given Data / Assumptions:

• We are talking about magnetic hard disks with multiple platters and heads.
• Access without repositioning heads means no seek—only rotational latency applies.
• The definition involves same-radius tracks across surfaces.

Concept / Approach:

A cylinder is the vertical alignment of tracks of equal radius on every platter surface. Because each head already sits over the correct radius, switching between heads to access another surface at the same track number requires no mechanical seek—only an electronic head switch and rotation.

Step-by-Step Solution:

Verification / Alternative check:

Filesystem layout strategies often place related data within the same cylinder group to reduce average seek time and improve throughput.

Why Other Options Are Wrong:

• Surface: refers to one side of one platter only.
• Cluster: filesystem allocation unit; unrelated to drive geometry.
• All of the above / None: do not fit the precise definition.

Common Pitfalls:

• Confusing logical filesystem terms (clusters) with physical geometry.
• Assuming SSDs share these terms; SSDs lack cylinders because there are no moving heads.

Final Answer:

Cylinder.