A hard disk drive (HDD), hard disk, hard disk drive, or fixed disk is an electromechanical data storage device that uses magnetic memory and one or more rigid fast rotating disks coated with magnetic materials to store and retrieve digital data. The disk is paired with a magnetic head, which is usually arranged on a moving actuator arm to read and write data on the surface of the disk. Data is accessed in random access, which means that individual data blocks can be stored and retrieved in any order. HDD is a non-volatile memory that retains stored data even when the power is off. Modern HDDs usually take the form of small rectangular boxes.
History Of Hard Disk
IBM launched in 1956 HDD is the main secondary storage device for general-purpose computers that began in the early 1960s. Hard drives maintain this position in the modern server and personal computer era, although mass-produced personal computing devices, such as mobile phones and tablets, rely on flash storage devices. In history, more than 224 companies produced HDDs, but after extensive industry consolidation, most of the equipment was manufactured by Seagate, Toshiba, and Western Digital. Hard drives dominate the amount of storage (exabytes per year) produced by servers. Despite the slow growth in production (calculated in EB shipments , sales revenue and unit shipments are declining because solid state drives (SSDs) have higher data transfer rates, higher areal storage densities, and better Reliability, and lower latency and access time
The two most common form factors for modern HDDs are 3.5-inch, for desktop computers, and 2.5-inch, primarily for laptops. HDDs are connected to systems by standard interface cables such as PATA (Parallel ATA), SATA (Serial ATA), USB or SAS (Serial Attached SCSI) cables.
Modern hard drives record data by magnetizing thin films of ferromagnetic material on both sides of the disk. Continuous changes in the direction of magnetization represent binary data bits. Read data from the disk by detecting the magnetization transition. User data is encoded using an encoding scheme, such as run-length-limited encoding, which determines how the data is represented by magnetic transitions. A typical HDD design consists of a spindle equipped with a flat circular disk, called a platter, used to store recorded data. The disc is made of non-magnetic material, usually aluminum alloy, glass or ceramic. They are coated with a shallow layer of magnetic material, usually 10-20 nanometers deep, and the outer layer is carbon as a protective layer. As a reference, the thickness of standard copy paper is 0.07-0.18 mm (70,000-180,000 nanometers) .
The factors that limit the time for accessing data on HDD are mainly related to the mechanical characteristics of rotating disks and moving heads, including:
- Seek time is a measure of the time it takes for the head assembly to travel to the disk track containing the data.
- The rotation delay occurs because the disk sector required when requesting data transfer may not be directly under the head. According to the statistical relationship that the average delay is one-half of the rotation period, the average rotation delay is shown in the table.
- The bit rate or data transfer rate (once the header is in the correct position) introduces a delay, which is a function of the number of transfer blocks; it is usually relatively small, but can be very long when transferring large continuous files.
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