Currently, the most widely-used storage device is the Hard Disk Drive (HDD), but its popularity is rapidly declining.
In an earlier column -- Digital Data Storage is Undergoing Mind-Boggling Growth -- we considered the mind-boggling growth of electronic data, which exceeded 10,000 exabytes or 10 zettabytes in 2016. By 2020, the volume of data is estimated to surpass 50 zettabytes. It's worth mentioning that there is only one prefix left, the "yotta," which was established at the 19th General Conference on Weights and Measures in 1991, before we run out of prefixes.
This follow-up column is co-authored by Ben Whitehead, who is a storage product specialist at Mentor Graphics. In this column we will discuss the evolution of data storage technologies and introduce the two predominant current and alternative storage technologies: the hard disc drive (HDD) and the solid state drive (SSD). Following this introduction, the focus of this column will be the HDD, its functions, and associated design verification methodology. In a future column, we will concentrate on the SSD, concluding with trends in data storage for the foreseeable future.
The history of electronic data storage evolved hand-in-hand with that of the computer. One could not exist without the other. After all, a computer needs storage to hold programs and data.
From the perspective of storage, programs and data are two sides of the same coin. They consist of strings of binary numbers that only computers can make sense of. Depending on how they are used, the storage requirements are rather different. When programs and data are in use concurrently, the media supporting them is called "main memory" or "primary memory" or just "memory." Conversely, when they are preserved for future use, the media supporting them is called "secondary memory" or just "storage."
Memory characteristics include fast data storing/retrieving capability, limited capacity, and higher cost compared to storage. On the other hand, storage characteristics comprise significantly larger capacity, but slower data storing/retrieving speed and lower cost than memory. Basically, storage holds far larger amounts of data at lesser cost per byte than memory. Typically, storage is two orders of magnitude less expensive than memory.
Unlike memory, storage can further be classified as online or offline. Figure 1 shows the main characteristics of memory and storage.
Figure 1. Requirements for memory and storage are different, depending on their characteristics, as shown here, including now obsolete technologies (Click Here for a larger image. Source: Lauro Rizzatti)
The technologies devised for implementing memory and storage that have evolved over time are remarkable examples of human ingenuity. Inventors have exploited mechanical, electromagnetic, electrostatic, electrical, optical, and semiconductor properties. A non-exhaustive list of memory/storage media includes: punched paper cards; punched paper tapes; delay lines (magnetostrictive wires or mercury delay lines); electrostatic memory tubes; charged capacitors; magnetic drums, tapes, cores, and discs; optical discs, and semiconductor chips. Some were very short-lived (e.g., electrostatic memory tubes), while others (e.g., magnetic cores) lasted for a few decades before being retired.
Today, memory is universally implemented using semiconductor chips. By comparison. storage is in the middle of a historical transition from magnetic discs to semiconductors, with the latter rapidly expanding and replacing their magnetic disc counterparts.
Progressively, memory and storage capacity grew from a few bytes to kilobytes, megabytes and gigabytes. Today, the capacity of a storage device -- even on a home computer -- is often measured in terabytes.
Storage devices encompass two parts: the media that stores the data and the controller that acts as a "traffic cop," supervising the flow of binary data in and out of the storage cells. The controller is the brain of a storage device. A poorly designed controller can quickly generate traffic congestion and slow down the computer's operations.
Hard Disk Drive (HDD)
At the time of this writing, the most popular storage device is the Hard Disk Drive (HDD), but its popularity is rapidly declining. The HDD has been around for three decades or so. Thirty years ago, there were many suppliers of hard drives. Over time, the industry saw massive consolidation that led to a virtual monopoly. Today, three big storage vendors survive: Seagate, Toshiba, and Western Digital, all of whom evolved through acquisitions of dozens of former big players, such as Conner Peripherals, Maxtor, and many others. Emblematic of this trend is the last acquisition of HGST (Hitachi Global Storage Technologies). Regulators split its products into two buckets: the 2.5-inch drives went to Toshiba, while the 3.5-inch drives went to Western Digital.
The three big companies own all the patents, the clean room technology, manufacturing robots, and so on. Basically, they monopolize the HDD industry. One could not acquire another company in the hard disk drive business -- not that any still exist -- without triggering the involvement of the regulators.
The main reason for this state of affairs is the enormous entry barrier that prevents small players from throwing in their hats. The money and engineering efforts required to create a modern HDD business are massive.