Disk storage
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Disk storage or disc storage is a general category of storage mechanisms, in which data are digitally recorded by various electronic, magnetic, optical, or mechanical methods on a surface layer deposited of one or more planar, round and rotating platters. A disk drive is a device implementing such a storage mechanism with fixed or removable media; with removable media the device is usually distinguished from the media as in compact disc drive and the compact disc. Notable types are the hard disk drive (which contain a non-removable disc), the floppy disk drive and its removable floppy disk, various optical disc drives and associated media.
Background
Musical and audio information was originally recorded by analog methods (see Sound recording and reproduction). Similarly the first video disc used analog recording. Analog recording has been mostly replaced by digital optical technology where the data is recorded in a digital format as optical information.
The first commercial disk storage device, that is the first commercial digital disk storage device, was the IBM RAMAC 350 shipped in 1956 as a part of the IBM 305 RAMAC computing system. Disk storage is now used in both computer storage and consumer electronic storage (e.g., audio CD and video DVD).
The random-access, low-density storage of disks was developed to complement the already used sequential-access high-density storage provided by magnetic tape. Vigorous innovation in disk storage technology, coupled with less vigorous innovation in tape storage, has reduced the density and cost per bit gap between disk and tape, reducing the importance of tape as a complement to disk.
Today disk storage devices typically have a single head that moves across a disk surface; earlier there were fixed head devices with multiple heads per surface but today they are no longer being manufactured. Movable head devices store more data per sensor and usually more per area of the medium. Fixed head devices avoid the seek time, while the head moves to the data.
Access methods
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Disk drives are block storage devices. Each disk is divided into logical blocks (collection of sectors). Blocks are addressed using their logical block addresses (LBA). Read from or writing to disk happens at the granularity of blocks.
Originally the disk capacity was quite low and has been improved in one of several ways. Improvements in mechanical design and manufacture allowed smaller and more accurate heads, meaning that more tracks could be used on each of the platters. Advancements in data compression methods created more information in each of the individual sectors, and this was instrumental in allowing drives to store smaller units of data. sectors of data, the smallest unit stored, were reduced and so less wasted space was created.
The drive stores data onto cylinders, heads and sectors. The sectors unit is the smallest size of data to be stored in a Hard Disk Drive and each file will have many sectors units assigned to it. The smallest entity in a CD is called a frame, which consists of 33 bytes and contains six complete 16-bit stereo samples (two bytes × two channels × six samples = 24 bytes). The other nine bytes consist of eight CIRC error-correction bytes and one subcode byte used for control and display.
The information is sent from the computer processor to the BIOS into a chip controlling the data transfer. This is then sent out to the hard drive via a multi-wire connector. Once the data is received onto the circuit board of the drive, it is translated and compressed into a format that the individual drive can use to store onto the disk itself. The data is then passed to a chip on the circuit board that controls the access to the drive. The drive is divided into sectors of data stored onto one of the sides of one of the internal disks. In the picture opposite we have two disk, this gives us 4 sides.
The controller chip determines available free space by listing sectors in a table of used and unused areas. This list is what determines where each part of a file is kept and where they are relative to track, sector and disk. Different file systems use different types of addressing systems such as FAT, NTFS, Joliet (ISO 9660) and UDF. Two copies of these lists are normally held and are used to ensure data integrity. If a problem occurs a disk check can be run and it compares these two lists to determine where the problem occurs and rebuilds the file structure from them. When a computer drive is formatted the files are not erased but rather the lists are simply deleted. The drive hardware will now treat all sectors as clean and overwrite them with new data.
The hardware on the drive tells the actuator arm where it is to go for the relevant track and the compressed information is then sent down to the head which changes the physical properties, optically or magnetically for example, of each byte on the drive, thus storing the information. A file is not stored in a linear manner, rather, it is held in the best way for quickest retrieval.
Mechanically, there are usually two types of motion: the constant rate rotation, which passes the data of a track sequentially under a read head, and the radial (side-to-side) head motion or seek, which selects the track. Rotation is faster than seek, so the logical blocks are related in simple ways to the physical tracks.
Interfaces
Disk drive interface is the mechanism/protocol of communication between the rest of the system and the disk drive itself. Different interface types include SCSI and SAS for enterprise or high-end disks, ATA (PATA), SATA for desktop disks.
Basic terminology
- Rotation - how the disks spin. Two techniques are common:
- Constant angular velocity (CAV) keeps the disk spinning at a fixed rate, measured in revolutions per minute (RPM). This means the heads cover more distance per unit of time on the outer tracks than on the inner tracks. This method is typical with computer hard drives.
- Constant linear velocity (CLV) keeps the distance covered by the heads per unit time fixed. Thus the disk has to slow down as the arm moves to the outer tracks. This method is typical for CD drives.
- Sector - an area of disk enclosed within a given central angle (a pie piece)
- Platter - an individual recording surface. In a hard disk drive we tend to find a set of platters and developments in optical technology have led to multiple recording layers on a single DVD's.
- Low level formatting - establishing the tracks and sectors.
- Track - the circle of recorded data on a single recording surface for a single arm position.
- Head - the device that reads and writes the information - magnetic or optical - on the disk surface.
- Arm - the mechanical assembly that supports the head as it moves in and out.
- Seek time - average time needed to move the head to a new position(specific track).
- Rotational delay - average time, once the arm is on the right track, before a head is over a desired sector.
- Interleave - the spacing between sectors. Sequential sectors were spaced on the media to enable the next sector to be in the correct position under the head once the host was ready to read it. At an interleave factor of 3:1, three full rotations would be required to read an entire track. Due to technological advances almost all hard disk drives since Compaq and Western Digital defined the AT Attachment standard have used an interleave factor of 1:1. Floppy disks are still usually interleaved.