Hard disk drives (HDDs) have been the predominant form of computer data storage since the 1960s. Despite competition from solid state drives in recent years, HDDs remain essential for high-capacity, cost-effective bulk storage. There are three main technologies used in modern HDDs:
Magnetic Recording
The recording surface of HDD platters is coated with a thin magnetic film. Data is stored by selectively polarizing tiny regions of the film to represent 1s and 0s. A read/write head floats just above the surface on an air bearing and magnetizes regions of the film to write data. It detects the magnetization of the regions to read the data back.
Early HDDs used longitudinal recording, where the magnetization was horizontal and parallel to the surface. Later drives adopted perpendicular recording, where magnetization is vertical. This allows for greater storage density. Magnetoresistive heads enabled higher precision reading and writing of the magnetic regions.
Key Features
- Magnetized regions represent binary data
- Read/write heads modify and detect magnetization
- Perpendicular recording enables greater density
Head Positioning
The read/write head must be positioned precisely over the desired track on the platter to read or write data. HDDs use a few key technologies for head positioning:
Voice Coils
Voice coils leverage magnetic fields to finely control head position. When current is applied, the coil interacts with the field to provide precise linear motion.
Servo Mechanism
Dedicated servo sectors on each track provide position feedback. By reading these sectors, the HDD can detect if the head is drifting off track and correct it.
Head Actuators
Rotary actuators pivot to move the arms containing the read/write heads across the platters. Higher performance HDDs may use dual actuators for faster head switching.
Data Encoding
Data stored on HDDs must be encoded in a way that can be reliably retrieved. Key encoding methods include:
Run Length Limited Encoding
RLL encoding limits long stretches of 1s or 0s to enable accurate clock recovery upon reading the data back.
Partial Response Maximum Likelihood
PRML uses advanced signal processing to extract stored data from the analog read signal. It enables greater density.
Low Density Parity Check
LDPC is an error correcting code that allows data to be recovered even with high noise levels.
Conclusion
Modern HDDs leverage magnetic recording, advanced servo control, and data encoding techniques to provide enormous, affordable data storage capacity. Though challenged by flash memory, HDD technology continues to advance and remains integral for cloud storage, data centers, NAS devices, and more. Key innovations like shingled magnetic recording, microwave assisted magnetic recording, and sealed helium-filled drives push HDD capabilities further.
| Technology | Description |
|---|---|
| Magnetic Recording | Stores data by selectively magnetizing tiny regions of a platter’s surface |
| Head Positioning | Precisely locates the read/write head over the desired track |
| Data Encoding | Encodes the binary data in a way that allows reliable storage and retrieval |
Hard disk drives have come a long way since the days of large, bulky, and slow units with severely limited capacity. Through ongoing innovations, HDDs continue to offer an unmatched combination of high capacity, performance, and low cost for mass data storage. While NAND flash has made gains in read/write speed and reliability, it remains considerably more expensive per gigabyte compared to HDDs.
For applications that demand the ultimate in speed, ruggedness, or miniaturization, solid state drives are now available as a high-performance alternative to traditional hard disk drives. However, HDDs will continue playing a major role across a broad range of computing and data storage applications for years to come. Thanks to steady technology improvements, hard drives can store more data in smaller spaces while also boosting durability and energy efficiency.
History of Hard Drive Development
The first commercial hard disk drive was introduced by IBM in 1956. The 350 RAMAC system contained fifty 24-inch platters and had a total storage capacity of 5 MB. Despite the substantial size and cost of these early drives, demand grew rapidly due to the dramatic increase in processing capabilities they enabled.
During the 1960s and 1970s storage capacities grew exponentially even as physical drive size shrank. By the late 1970s, small form factor hard disks with capacities in the 10s of MB were being incorporated into microcomputer systems and data terminals.
The 1980s brought further miniaturization along with the first 2.5-inch hard drives for mobile applications. Increased adoption of networked personal computers fueled rapid development, and 1GB+ capacities were reached by the early 1990s.
The rise of the internet brought explosive storage growth in the late 1990s and into the new millennium. Perpendicular recording, tunnel magnetoresistance heads, and other advances pushed densities high enough for multi-terabyte 3.5-inch HDDs available today.
Major Milestones
- 1956 – IBM 350 RAMAC – 5 MB capacity
- 1962 – IBM 1311 – 2.6 GB capacity
- 1979 – Microcomputer HDDs – 10s of MB
- 1983 – Rodime RO352 – First 2.5-inch HDD
- 1991 – Integral Peripherals Mustang 1820 – First 1 GB HDD
- 2000s – Rapid density growth continues
- 2016 – Western Digital – First 10 TB HDD
Hard drive capacities had reached around 10-20GB by the late 1990s. This was sufficient for most desktop computer storage needs at the time. The rise of digital photography, music, and video in the early 2000s kickstarted a new wave of rapid growth. In 2005, the first 1TB hard drives were introduced, representing a 50x jump in capacity from just a few years prior.
Perpendicular recording and other advances allowed densities to scale up dramatically through the 2000s. Hard drives with capacities numbering in the teens of TBs are now available. Data center and NAS storage applications are fueling demand for ever-greater storage capacities in compact HDD form factors.
Understanding Recording Density
Areal density is one of the key factors that determines a hard drive’s capacity. It represents the amount of data that can be stored per square inch of platter area and is measured in gigabits per square inch (Gb/in2). The greater the recording density, the more data can be packed onto a disk.
Early HDDs had densities of just a few megabits per square inch. Densities passed 100 Gb/in2 in the early 2000s, reached around 500 Gb/in2 by 2010, and are now approaching 1 Tb/in2 in cutting-edge drives. Densities are increased by improvements in magnetic recording physics along with advances in error detection and correction.
Increasing Areal Density
Major factors enabling continued density growth include:
- More precise write heads and magnetic media
- Shingled magnetic recording
- Microwave-assisted magnetic recording
- Two-dimensional magnetic recording
- Helium-filled drives reduce turbulence
Hard drive engineers continue finding ways to pack more data onto spinning platters. Ongoing innovations in recording physics combined with error correction and signal processing will enable densities to scale up even further in the future.
Common Hard Drive Form Factors
While drives have shrunk dramatically over the decades, common form factors have emerged based on usage models. The main HDD form factors are:
3.5 inch
– Used for desktop computers and servers
– Typically 500GB – 10TB+ capacities
2.5 inch
– Used for laptops and certain specialized applications
– Typically 500GB – 2TB capacities
1.8 inch
– Used in portable external HDDs and ultracompact devices
– Typically 320GB – 1TB capacities
1 inch
– Used in compact mobile applications
– Typically 4GB – 256GB capacities
Larger form factors like 3.5-inch and 2.5-inch drives are designed for performance and capacity demands in desktop computers, servers, NAS devices, and the like. The smaller form factors prioritize low power draw and portability for mobile applications.
Comparison of HDDs vs SSDs
Solid state drives (SSDs) have become a popular alternative to traditional hard disk drives in some applications. SSDs store data in integrated flash memory chips rather than on rotating platters. Compared to HDDs, SSDs have notable advantages as well as disadvantages.
SSD Advantages
- Faster read/write speeds
- Lower latency
- Better reliability and shock resistance
- Lower power consumption
- Lighter weight and smaller form factors
SSD Disadvantages
- Higher cost per gigabyte
- Lower capacities (improving rapidly)
- Wear out from repeated writes
For applications demanding top speed, ruggedness, or miniaturization, SSDs are often the best choice despite the higher cost. However, HDDs still offer unbeatable capacity for lower cost bulk storage. The two technologies will likely co-exist for many more years serving complementary roles.
Role of HDDs in Data Centers
Data centers operate massive computer systems to provide cloud services at a global scale. Cost-effective, high-capacity storage is vital for the proliferating amount of data generated. HDDs store the vast majority of data center content because of capacious storage at low cost.
Large scale deployments favor 3.5-inch and 2.5-inch enterprise HDDs optimized for performance, reliability, and density. Data center HDD unit shipments experience strong and steady growth to feed the planet’s insatiable demand for digital content storage.
Ideal for Large-Scale Storage
- Massive aggregated capacity
- Cost efficient storage at scale
- Time-tested technology
- Continued density improvements
While high performance flash storage has a role, HDDs will continue providing the bulk repository holding humanity’s exponentially growing trove of digital content. Developing higher capacity drives for cloud storage and other data center applications remains a key area of HDD innovation.
Conclusion
For over six decades, hard disk drive technology has continually evolved to provide greater and greater storage capacity within shrinking physical packages. HDDs have enabled a digital content explosion powering the information age.
Three core technologies work together for the magic of persistent data storage on a rotating disk platter that can be accessed in milliseconds:
- Magnetic recording – Stores data by flipping magnetic domains on a platter’s surface.
- Head positioning – Uses voice coils and actuators to locate the R/W heads.
- Data encoding – Encodes the binary data for reliable storage and retrieval.
Remarkable engineering has packed greater and greater density of magnetic domains onto platter surfaces through the decades. While solid state drives are now viable for some applications, HDDs continue offering compelling advantages of cost, capacity, and time-proven technology.
Ongoing R&D aims to push HDD areal densities toward 1 Tb/in2 and beyond. New technologies like shingled magnetic recording, two-dimensional magnetic recording, and microwave assisted recording will help squeeze more capacity from spinning disks. Hard drives will continue playing an indispensable, pivotal role across many computing applications going forward.