Disk storage: Wikis

  
  

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Disk storage or Disc storage is a general category of a computer storage mechanisms, in which data is recorded on planar, round and rotating surfaces (disks, discs, or platters). A disk drive is a peripheral device used to record and retrieve information. Main implementations are hard disks, floppy disks and optical discs. Nowadays the term disk storage almost exclusively refers to hard disk storage.

Contents

Background history

In the late 1940s information from the Manchester Mark 1 was stored externally onto a rotating drum, created in Manchester, England by Prof. F.C. Williams and J.C. West. The drum was coated with Nickel.[1] The magnetic drum, or "wheel" as it was first called, had a set of parallel magnetic tracks around the circumference of the drum forming a circle for each with a head for each of the tracks. Each of these tracks was capable of holding 2560 bits, or 2 pages. The drum had to be carefully synchronised with the main memory store of the computer and was capable of reading data twice as fast as it could write it.

Ferranti Ltd., who were developing their business machines from the work done in Manchester, took the prototype drum and improved it, making the reading heads and mechanical parts of the drum much more accurate and stable. This external unit was a 32 page capacity drum backing store, with 2 pages per track, and revolved once per 30 milliseconds.

The scientists in Manchester then adopted the improved version for use on the 'Mark 1 Final Specification' due to its better performance. Ferranti used this in the Ferranti Mark 1 commercially available computer produced at their works in Moston, Manchester. From the Ferranti Mark 1's sales book of the time:

'Storage capacity. High speed -- 10,000 binary digits on cathode ray tubes; equivalent to about 3,000 decimal digits. Magnetic drum -- 650,000 binary digits; equivalent to about 15,000 twelve-digit decimal numbers or 30,000 six-digit decimal numbers.'[2]

Then scientists at IBM in San Jose, California expanded on the rotating drum method. The drum was superseded by disks, because of their lower mass and inertia. Rey Johnson, an inventor who worked for IBM for many years, is said to be the "father" of the disk drive.

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.

These were movable head devices, usually disks, and fixed head devices, usually drums. Movable head devices store more data per magnetic sensor and usually more per area of the medium. Fixed head devices avoid the seek time, while the head moves to the data. Fixed head devices have not been common since integrated circuit random access memory was developed. So the usual storage devices of, for example, an IBM 360 were discrete transistor registers, magnetic core random access memory, fixed head drums, movable head disk packs (several disks with the heads connected mechanically), and magnetic tape, in order of increasing time to access a random data element.

Audio recordings

In musical and audio data storage, the first devices were also drum shaped, called phonograph cylinders, which were popularized by Thomas Edison. In the 1910s these were replaced as the dominant medium of sound recording by analogue disc records, commonly called gramophone records (in British English) or phonograph records (in American English). From the 1950s through the 1980s, audio recordings were also done on magnetic tape media of several types, although the vinyl record remained the most popular medium for home use. These were mostly replaced by compact disc technology, where the data is recorded in a digital format as optical information. This compact disc technology has been widely accepted, and data storage, using writable compact disks or CD-R devices is very common.

Access methods

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 and each file will have many sectors units assigned to it.

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. For example, Vista uses the NTFS system, whereas Windows 98 systems used the FAT system. Two copies of these lists are normally held and are used to ensure data integrity. If a problem occurs with a hard drive, a disk check can be run and it compares these two lists to determine where the problem occurs and rebuild the file structure from them. When a computer drive is formatted, the data are not erased, the lists are simply deleted and so the drive hardware will treat all sectors as clean and overwrite them with new data.

The hardware on the drive now 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 magnetic properties of each byte on the drive, thus storing the information. The average modern hard drive has 16 heads, 4 on each end of an actuator arm, and four arms, one arm per side of the disk. This gives a total of 16 heads, each of which can write information onto the disk platter itself. 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.

Crashing

Since a magnetic dipole field decreases rapidly with distance from the magnetic material, the space between the head and medium must be controlled with more precision than the thermal expansion of parts of the disk drive. The head, therefore "flies" or is hydrodynamically lubricated by air. That is, the air pulled along by the disk forces the head away from the disk surface. When the disk stops, the head must either "land" or be pulled away. Defects, wear or foreign objects such as dust, can distort the disk and head surfaces, so they make contact and damage each other further. When these elements build up over time, there is a chance that a "head crash" will occur. This results in lost data and an inoperable device. Head crashes are one of the reasons that important data must be backed up on some other device (often audio or video media). When removable hard disks were used, disc damage was contagious, since either the head or the disk could propagate the damage.

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 disk (since confusingly, what is now commonly called a single hard disk is in fact a set of disks)
  • 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. Since early hard disks had only enough buffer space to read one sector at a time, sequential sectors were physically 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. Almost all hard disks since Compaq and Western Digital defined the AT Attachment standard have however used an interleave factor of 1:1. Floppy disks are still usually interleaved.

See also

Notes

  1. ^ Dr T Kilburn: The University of Manchester Universal High-speed Digital Computing Machine, Nature, Vol 164, page 684 October 22 1949. Macmillan Magazines Limited, 1949.
  2. ^ Ferranti Ltd.: The Manchester Universal Electronic Computer, Ferranti Ltd., August 1952.







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