Uncovering Disk History: From IBM 334 to Modern 4K Alignment

Introduction

This article, prepared by the HULK virtualization team’s storage sub‑team, summarizes the historical evolution of disks. Storage is a critical component in cloud computing, yet many focus only on usage without understanding its origins and basic concepts.

Early Hard Disk – The Winchester

In 1973 IBM developed the IBM 334, a new hard‑disk that used several coaxial metal platters coated with magnetic material. The rotating platters and a movable magnetic head were sealed in a single enclosure, and IBM called this the Winchester disk.

The Winchester is the ancestor of today’s mechanical hard drives. Over more than 40 years, computer hardware has changed dramatically and capacities have grown, but the basic mechanical structure remains largely the same.

Structure Composition

A hard disk consists of three main parts: the platters (where data is stored), the motor that spins the platters, and the actuator arm with the read/write heads. Additional components include control chips and the disk enclosure.

CHS Geometry

CHS stands for Cylinder, Head, Sector. It is a traditional way to calculate disk capacity.

Each platter is divided into concentric circles called tracks (cylinders). Tracks are not packed tightly; there is spacing to avoid magnetic interference. A track is further divided into sectors, the smallest addressable units.

Early disks used a fixed number of sectors per track, wasting space on outer tracks. Modern disks vary the sector count per track, but capacity is often estimated using an average sector count.

Cylinders are formed by stacking tracks of the same radius across all platters. Heads are the read/write elements; each usable surface of a platter has its own head.

Analogy: imagine a stack of CDs. Each CD’s circular track is a magnetic track, a small segment of that track is a sector, and a stack of CDs forms a cylinder. The needle on each CD represents a head.

Inspecting Disk Geometry with fdisk

Running fdisk -lu /dev/sda on a Linux system yields the following output:

<code>fdisk -lu /dev/sda</code>

<code>Disk /dev/sda: 214.7 GB, 214748364800 bytes</code>

<code>160 heads, 24 sectors/track, 109226 cylinders, total 419430400 sectors</code>

<code>Units = sectors of 1 * 512 = 512 bytes</code>

<code>Sector size (logical/physical): 512 bytes / 512 bytes</code>

<code>I/O size (minimum/optimal): 512 bytes / 512 bytes</code>

<code>Device Boot      Start        End    Blocks   Id  System</code>

<code>/dev/sda1   *       2048   419427839 209712896  83  Linux</code>

This indicates the disk has 160 heads, 24 sectors per track, 109 226 cylinders, and a total of 419 430 400 sectors. Each sector is 512 bytes.

Using CHS, the capacity can be calculated as:

<code>CHS = 109226 × 24 × 160 × 512 bytes = 214.7 GB</code>

Sector Size Evolution

Historically, a sector size of 512 bytes became an industry convention because it balanced read/write efficiency and space utilization. However, as disk capacities grew, 512‑byte sectors became inefficient for large files.

Hard‑disk manufacturers such as Western Digital introduced the Advanced Format , using 4096‑byte (4 KB) physical sectors while keeping 512‑byte logical sectors for compatibility.

This change created challenges because many software layers—BIOS, bootloaders, operating system kernels, file systems, and disk utilities—assume 512‑byte sectors. Switching to 4 KB sectors can break boot processes and requires updates across the software stack.

4K Alignment

To mitigate performance penalties, manufacturers often present 4 KB physical sectors as eight 512‑byte logical sectors, making the disk appear 4 KB‑aligned to the OS. Linux typically writes in blocks of eight sectors (4096 bytes). If a partition does not start on a sector number divisible by 8, writes may span two physical sectors, reducing performance.

In the fdisk output above, the first usable sector is 2048, which is divisible by 8, indicating the disk is 4 KB aligned.

The first 2048 sectors (1 MiB) are reserved for the partition table (MBR) and bootloader (GRUB). Sector 0 holds the MBR, while sectors 1–2047 are used by GRUB.

Conclusion

Understanding the historical and technical details of disk geometry, sector sizes, and alignment helps in making informed decisions about storage configuration and performance optimization.