Magnetic disks (Hard Disk Drives - HDDs) are fundamental components of computer systems, providing non-volatile, large-capacity secondary storage. Their physical and logical organization directly impacts how data is stored, accessed, and managed by the operating system.

Physical Structure of a Hard Disk Drive:

• Platters: A disk drive typically contains one or more rigid, circular metal or glass platters, each coated with a magnetic material. Data is recorded magnetically on both surfaces of each platter (except sometimes the very top and bottom surfaces, depending on the design).
• Spindle: All platters are rigidly mounted on a central spindle that rotates at a very high and constant speed (e.g., 5,400 RPM for laptops, 7,200 RPM for desktops, 10,000-15,000 RPM for servers).
• Read/Write Heads: For each platter surface, there is a dedicated read/write head. These heads are tiny electromagnets that float on a thin cushion of air just micrometers above the rotating platter surface, without actually touching it.
• Arm Assembly (Actuator Arm): All read/write heads are mounted on a single arm assembly that pivots across the platters. This arm moves the heads radially inward and outward across the disk surfaces.
• Tracks: Each platter surface is logically divided into many concentric rings called tracks. Data is stored along these tracks. Tracks closer to the center are typically denser in terms of bits per inch, but the total number of bits per track can be constant across the disk due to zone bit recording (ZBR).
• Sectors: Each track is further subdivided into smaller, arc-shaped units called sectors. A sector is the smallest unit of data that can be read from or written to the disk by the controller (common sizes are 512 bytes or 4 KB). Each sector typically includes a header, the data area, and a trailer containing Error-Correcting Code (ECC) for data integrity.
• Cylinders: A cylinder is a conceptual stack of all tracks that are at the same radial distance on all platter surfaces. For a given cylinder, all read/write heads are positioned over their respective tracks simultaneously. This is important for reducing seek time, as data can be read from multiple surfaces without moving the arm.

Logical Block Addressing (LBA):

• Modern disks simplify addressing for the operating system by using Logical Block Addressing (LBA). Instead of requiring the OS to specify the physical cylinder, head, and sector (CHS) for data, the disk controller presents the entire disk as a single, one-dimensional array of logical blocks.
• The first logical block is 0, the next is 1, and so on, up to the total number of blocks on the disk.
• When the OS requests data from a specific logical block number, the disk controller performs the internal translation from the logical block number to the physical cylinder, head, and sector address. This abstraction makes disk management easier for the OS and allows manufacturers to change physical geometry without affecting the OS.

Disk I/O requests involve mechanical movements: the arm must move to the correct cylinder (seek time), and the desired sector must rotate under the head (rotational latency). Seek time is usually the most significant component of disk access time. Disk scheduling algorithms aim to minimize the total seek time for a sequence of pending I/O requests, thereby improving disk throughput and reducing average response time.

Let's use an example to illustrate the algorithms.

Disk I/O Requests Example:

Disk with 200 cylinders (0-199). Current head position: 53.
Pending Disk Request Queue (in order of arrival): 98, 183, 37, 122, 14, 124, 65, 67.