A hard disk drive (HDD) or hard disk is a type of computer storage device used for storing digital information.
Hard disks were first introduced by IBM in 1956 for use with their 305 RAMAC computer system. Since then, they have evolved to become a common non-volatile storage device found in computers, servers, and consumer electronics.
The main components of a hard disk include platters, read/write heads, spindle, actuator arm, and controller circuitry. The platters are made of metal or glass that are coated with magnetic material to store data. The read/write heads are responsible for writing data to the platters and reading data back. The spindle rotates the platters at high speeds while the actuator arm moves the read/write heads across the platters. The controller coordinates all of these components.
The purpose of the hard disk is to store large amounts of digital data including the operating system, applications, files and media. Unlike volatile RAM, hard disks retain data even when the power is turned off. Hard disks offer much larger storage capacity compared to solid state drives and provide cost-effective storage solutions.
In summary, hard disks are non-volatile, high capacity storage devices that store and retrieve digital information using magnetic recording heads and spinning magnetic platters. They are a fundamental component of modern computers.
Magnetic Storage
Hard disks store data on magnetized platters. Data is stored in tiny magnetic patches called bits. Platters are made of a non-magnetic material like aluminum or glass and are coated on both sides with a very thin layer of magnetic material.
The surface of each platter is divided into billions of tiny areas called sectors. Sectors are located along concentric circles called tracks. Each track is assigned a unique track number or address, allowing the hard disk head to accurately locate data. The head reads or writes data as the platters spin at very fast speeds of up to 15,000 rpm in desktop hard drives.
Data is written by the read/write head which generates a magnetic field to align magnetic particles into bits along the tracks. A bit’s magnetic polarity represents either a 1 or 0. Bits can be rewritten by reversing their magnetic alignment. This allows hard disks to be rewritten multiple times.
Hard disks have grown enormously in capacity. Early hard disks stored only a few megabytes, while modern consumer hard drives store up to 16 terabytes. This vast storage capacity comes from cramming more bits onto each platter by making bits smaller and packing tracks closer together.
Source: https://www.britannica.com/technology/hard-disk
Read/Write Heads
The read/write heads are the components in a hard drive that read and write data onto the spinning platters. Their purpose is to accurately record and retrieve the binary data from the magnetic medium coating the platters (Tom’s Hardware, 2008). The heads float just above the surface of the platters on an air bearing and move in and out across the radius of the platters to access data tracks (ACS Data, n.d.).
The read/write heads contain electromagnets that generate magnetic fields to magnetize tiny spots on the platter in one direction or the other to represent 0s and 1s. To write data, the head receives an electrical signal that polarizes the electromagnet, and the magnetic field produced magnetizes a tiny spot on the platter as it spins past the head. To read data, the head detects the magnetization of the spots passing under it and generates an electrical signal based on the magnetic flux. The signal is then decoded into 1s and 0s (Data Recovery Tools, n.d.).
The heads are attached to actuator arms driven by a voice coil that can quickly and precisely position the heads over the correct track on the platter to access data. The heads float incredibly close to the platter surface, with clearance smaller than the thickness of a human hair, in order to increase storage density (Tom’s Hardware, 2008).
Spindle/Platters
The spindle is the central shaft in the hard disk that rotates the platters at high speeds, typically 5,400 to 15,000 rpm. The platters are the circular disks that are stacked on the spindle and coated with a magnetic recording material.
The purpose of the spindle and platters working together is to spin the platters rapidly so that the read/write heads can access data quickly from anywhere on the disks. As the platters spin, the heads float nanometers above the surface on a cushion of air, allowing them to move in and out to different tracks.1 The faster the platters spin, the faster data can be accessed.
Hard disks contain one or more platters stacked on the spindle, with data stored on both sides of each platter. More platters provide more total storage capacity in the same form factor.
Cylinders
Tracks are concentric circles on each platter surface that store data. Tracks are grouped together to form cylinders. A cylinder is comprised of all the tracks located at a specific location across all the platters in the hard drive. So a cylinder grouping contains one track from each platter surface that is vertically in line with each other. This allows the read/write heads to move in unison to a specific cylinder to access the data on all surfaces simultaneously (Source).
Having the tracks lined up in cylinders allows the hard drive head actuator to switch tracks with only one movement across all surfaces. This makes data access much faster compared to accessing each track individually. The number of cylinders depends on the total number of tracks per platter and the number of platters in the hard drive.
Spinning Speed
Hard disk drives contain spinning platters that rotate at a certain speed measured in revolutions per minute (RPM). The RPM determines how fast data can be read from or written to the drive. Common RPM speeds for consumer hard drives include:
- 5400 RPM – Most common in laptops and budget desktops. Offers decent performance for everyday computing.
- 7200 RPM – Faster than 5400 RPM. Common in desktop PCs. Provides snappier load times for applications and files.
- 10,000-15,000 RPM – Found in high performance server drives. Very fast but generates more heat and noise.
Higher RPM leads to lower latency and quicker data access. However, some modern drives use caching and optimizations to boost performance beyond what RPM alone would indicate. Still, 7200 RPM drives tend to outperform 5400 RPM models for most everyday tasks [1]. For the best experience, match the RPM to your usage – prioritize 7200 RPM for gaming rigs and production workstations.
Caching
Caching is an important aspect of hard drive performance. The purpose of the cache is to store frequently accessed data in a small amount of faster memory, improving overall performance (Hard Disk Caching Support in CD/DVD Server). When data is requested from the hard drive, it first checks the cache to see if the data is there. If it is, the data can be returned much faster from the cache than having to physically locate it on the actual hard disk platters. This improves read speeds and overall performance.
The cache acts as a buffer between the CPU and the physical storage, storing copies of frequently used data. Having a larger cache can significantly improve performance, as more data can be stored and served from the fast cache memory rather than having to access the hard disk itself (Caching options). Hard disk caching behaves similarly to general memory caching in computers.
Overall, caching plays a crucial role in improving hard drive performance by reducing average access times. Storing commonly used data in fast cache memory allows the hard drive to operate faster than if it had to physically locate all data on the disk platters (Buffer and Cache Memory in Linux).
Disk Controller
The disk controller is an essential component of a hard disk drive. Its primary function is to translate instructions received from the computer into commands the hard disk drive can understand (Techopedia, para. 1). The controller acts as the interface between the disk drive and the rest of the computer system (Wikipedia, para. 1).
The controller receives read and write requests from the processor and converts them into electrical signals to control the mechanical operation of the drive. When data is read, the controller directs the read/write head to move to the correct track and sector. It then amplifies the signal from the disk, detects and corrects any errors, and sends the data to the processor. For writing data, the controller receives the data from the processor, encodes it, and sends electrical signals to the read/write head to magnetize the disk surface (Techopedia, para. 2).
Without the controller, the processor would not be able to communicate with the hard drive. It enables the high-speed, accurate reading and writing of data on the hard disk that is essential for modern computer operation.
Form Factors
Hard disk drives come in a variety of form factors that refer to the physical size and shape of the drive.
Some common form factors include (https://www.datarecoverytools.co.uk/data-recovery-vocabulary/vocabulary-f-j/form-factors-of-hard-disk-drive/):
- 3.5-inch – The most popular size for desktop computers.
- 2.5-inch – Used mainly in laptops.
- 1.8-inch – Found in some ultraportable laptops.
- 1-inch – Used in small devices like MP3 players.
Standardized form factors allow for interchangeability between different manufacturers and devices. They also help determine capacity limits. For example, 3.5-inch drives max out around 10TB for consumer models while 2.5-inch drives are typically lower capacity.
As capacities increase, new form factors may emerge to provide more storage space. There is discussion that shift to a new form factor may be needed to continue advancing hard disk capabilities (https://www.itprotoday.com/storage/storage-shift-time-may-be-right-new-form-factor).
Conclusion
In summary, hard disks store data magnetically on spinning platters accessed by read/write heads. The platters are organized into tracks and sectors to precisely locate data. Hard disks have continued to advance over the decades, with increases in capacity and speed. Current trends point to continued growth in capacity as drive sizes increase and bit density improves through new technologies like shingled magnetic recording and microwave assisted magnetic recording. Speeds should also continue to improve with advances in caching and the controller. Form factors may continue to shrink as well, with m.2 and other small drive designs gaining popularity for some applications. Though solid state drives are emerging as an alternative, hard disk drives will likely continue as a cost-effective storage solution into the foreseeable future.
