Disk storage
Rotating-disk data storage using magnetic, optical, or mechanical recording.
Disk storage, sometimes called drive storage, works by spinning a disk and using electronic, magnetic, optical, or mechanical changes to its surface to record data. The device that does this is a disk drive. Common types include hard disk drives (HDDs), which have one or more rigid, non-removable platters; floppy disk drives (FDDs) and their removable floppy disks; and optical disc drives (ODDs) along with their corresponding optical discs. The spellings "disk" and "disc" are used interchangeably, except where trademarks require one form—for example, the Compact Disc logo. The choice often comes down to historical convention, such as IBM's use of "disk" starting in 1956 with the IBM 350 disk storage unit.
Audio was first recorded using analog methods, and the earliest video discs also used analog recording. In music, analog recording has largely given way to digital optical technology, which stores data in a digital format using light. The first commercial digital disk storage device was the IBM 350, released in 1956 as part of the IBM 305 RAMAC system. Disks offered random-access, low-density storage, complementing the sequential-access, high-density storage of magnetic tape drives. Strong innovation in disk technology, alongside slower progress in tape technology, has narrowed the gap in cost per terabyte between the two, though the total cost of owning data on disk—including power and management—remains higher than for tape. Today, disk storage is used in computers and consumer electronics, such as audio CDs and video discs (VCD, DVD, and Blu-ray).
Modern disks store data in fixed-length blocks called sectors, which range from a few hundred to many thousands of bytes. Gross disk drive capacity is calculated by multiplying the number of disk surfaces by the number of blocks per surface by the number of bytes per block. Older IBM CKD drives used variable-length blocks called records; record length could differ between disks and even on the same disk. Capacity dropped as record length shrank because of the gaps required between blocks.
Digital disk drives are block storage devices. Each disk is split into logical blocks (collections of sectors), addressed by their logical block addresses (LBAs). Reads and writes happen at the block level. Early disk capacity was low, but improvements came in several ways.
- First commercial digital disk storage de
- IBM 350
- Year shipped
- 1956
- Part of system
- IBM 305 RAMAC computing system
- Traditional sector size older hard drive
- 512 bytes
- Newer advanced format drive sector size
- 4 KB
- Optical media sector size
- 2 KB
- Example rotational latency for 7200rpm d
- 4.17 milliseconds
Lore & Background
Audio information was originally recorded by analog methods, and the first video disc used an analog recording method. In the music industry, analog recording has been mostly replaced by digital optical technology, where data is recorded in a digital format with optical information. The first commercial digital disk storage device was the IBM 350, which shipped in 1956 as part of the IBM 305 RAMAC computing system. The random-access, low-density storage of disks was developed to complement the already used sequential-access, high-density storage provided by tape drives using magnetic tape.
Data on modern disks is stored in fixed length blocks, usually called sectors, varying in length from a few hundred to many thousands of bytes. In certain legacy IBM CKD drives, data was stored on magnetic disks with variable length blocks called records; capacity decreased as record length decreased due to necessary gaps between blocks. The drive stores data onto cylinders, heads, and sectors. The sector unit is the smallest size of data stored in a hard disk drive, while the smallest entity in a CD is called a frame, consisting of 33 bytes.
Mechanically, there are two motions: rotation of the disks and side-to-side motion of the head across the disk. Two types of disk rotation methods exist: constant linear velocity (used mainly in optical storage) and constant angular velocity (used in HDDs, standard FDDs, a few optical disc systems, and vinyl audio records). Storage devices focused on holding computer data use concentric tracks, while optical audio and video discs use a single spiral track.
Reader's Guide
Disk storage is now used in both computer storage and consumer electronic storage, including audio CDs and video discs such as VCD, DVD, and Blu-ray. The interface is the mechanism of communication between the system and the drive; desktop and mobile computers typically use ATA (PATA) and SATA interfaces, while enterprise systems and high-end storage devices use SCSI, SAS, and FC interfaces in addition to some SATA. Vigorous innovation in disk storage technology, coupled with less vigorous innovation in tape storage, has reduced the difference in acquisition cost per terabyte between disk storage and tape storage; however, the total cost of ownership of data on disk including power and management remains larger than that of tape. The spelling disk and disc are used interchangeably except where trademarks preclude one usage, as with the Compact Disc logo; the choice of a particular form is frequently historical, as in IBM's usage of the disk form beginning in 1956 with the IBM 350 disk storage unit.
Did You Know?
- Data on modern disks is stored in fixed length blocks called sectors; older hard drives traditionally used 512-byte sectors, while newer advanced format drives use 4 KB sectors.
- Optical audio and video discs use a single spiral track starting at the innermost point and flowing continuously to the outer edge, unlike computer storage devices that use concentric tracks.
- The smallest entity in a CD is called a frame, consisting of 33 bytes containing six complete 16-bit stereo samples plus error-correction and subcode bytes.
Frequently Asked Questions
What is disk storage?
Disk storage is a rotating-media technology that records data by making magnetic, optical, or mechanical changes to the surface of a spinning disk. The hardware that reads and writes those changes is called a disk drive.
When was the first commercial digital disk storage shipped?
IBM introduced the IBM 350 in 1956 as part of the IBM 305 RAMAC computing system, making it the world's first commercially available digital disk storage device.
What are the main categories of disk storage?
The three common families are hard disk drives (HDDs) with rigid non-removable platters, floppy disk drives (FDDs) that use removable floppy disks, and optical disc drives (ODDs) paired with optical discs.
How does a disk drive actually record data?
A motor spins the disk while a read/write head alters the surface using magnetic, optical, or mechanical means, and those surface changes are later interpreted as stored bits.
What sector sizes do different disk-storage media use?
Older hard drives traditionally used 512-byte sectors, while newer advanced-format drives moved to 4 KB sectors; optical media, by contrast, standardised on 2 KB sectors.
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