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Saturday, November 20, 2010

Types of Computer Hard Disks

Hard disk types

A hard disk drive (hard disk/hard drive/HDD) is a non-volatile storage device for digital data. It features rotating rigid platters on a motor-driven spindle within a protective enclosure. Data are encoded magnetically by read/write heads that float on a cushion of air above the platters.

Hard disk manufacturers quote disk capacity in SI-standard powers of 1000, wherein a terabyte is 1000 gigabytes and a gigabyte is 1000 megabytes. With file systems that report capacity in powers of 1024, available space appears somewhat less than advertised capacity.

The first HDD was invented by IBM in 1956. They have fallen in cost and physical size over the years while dramatically increasing capacity. Hard disk drives have been the dominant device for secondary storage of data in general purpose computers since the early 1960s.[5] They have maintained this position because advances in their areal recording density have kept pace with the requirements for secondary storage.[5] Form factors have also evolved over time from great standalone boxes to today's desktop systems mainly with standardized 3.5-inch form factor drives, and mobile systems mainly using 2.5-inch drives. Today's HDDs operate on high-speed serial interfaces; i.e., serial ATA (SATA) or serial attached SCSI (SAS).

1. IDE: Integrated Drive Electronics. IDE drives are also known as PATA drives (Parallel advance technology attachment)
2. SATA: Serial advance technology attachment
3. SCSI: Small Computer System Interface. SCSI is pronounced as skuzzy.
4. SAS: Serial Attached SCSI

Integrated Drive Electronics (IDE): - Parallel ATA (PATA), originally ATA, is an interface standard for the connection of storage devices such as hard disks, solid-state drives, floppy drives, and CD-ROM drives in computers. The standard is maintained by X3/INCITS committee. It uses the underlying AT Attachment (ATA) and AT Attachment Packet Interface (ATAPI) standards.

The current Parallel ATA standard is the result of a long history of incremental technical development, which began with the original AT Attachment interface, developed for use in early PC AT equipment. The ATA interface itself evolved in several stages from Western Digital's original Integrated Drive Electronics (IDE) interface. As a result, many near-synonyms for ATA/ATAPI and its previous incarnations exist, including abbreviations such as IDE which are still in common informal use. After the market introduction of Serial ATA in 2003, the original ATA was retroactively renamed Parallel ATA.

Parallel ATA only allows cable lengths up to 18 in (457 mm). Because of this length limit the technology normally appears as an internal computer storage interface. For many years ATA provided the most common and the least expensive interface for this application. By the beginning of 2007, it had largely been replaced by Serial ATA (SATA) in new systems.

Serial advance technology attachment(SATA): - Serial ATA (SATA or Serial Advanced Technology Attachment) is a computer bus interface for connecting host bus adapters to mass storage devices such as hard disk drives and optical drives. Serial ATA was designed to replace the older ATA (AT Attachment) standard (also known as EIDE). It is able to use the same low level commands, but serial ATA host-adapters and devices communicate via a high-speed serial cable over two pairs of conductors. In contrast, the parallel ATA (the re-designation for the legacy ATA specifications) used 16 data conductors each operating at a much lower speed.

SATA offers several advantages over the older parallel ATA (PATA) interface: reduced cable-bulk and cost (reduced from 80 wires to seven), faster and more efficient data transfer, and hot swapping.

The SATA host adapter is integrated into almost all modern consumer laptop computers and desktop motherboards. As of 2009[update], SATA has replaced parallel ATA in most shipping consumer PCs. PATA remains in industrial and embedded applications dependent on Compact Flash storage although the new CFast storage standard will be based on SATA.

Small Computer System Interface, or SCSI (pronounced skuzzy): - It is a set of standards for physically connecting and transferring data between computers and peripheral devices. The SCSI standards define commands, protocols, and electrical and optical interfaces. SCSI is most commonly used for hard disks and tape drives, but it can connect a wide range of other devices, including scanners and CD drives. The SCSI standard defines command sets for specific peripheral device types; the presence of "unknown" as one of these types means that in theory it can be used as an interface to almost any device, but the standard is highly pragmatic and addressed toward commercial requirements.

SCSI is an intelligent, peripheral, buffered, peer to peer interface. It hides the complexity of physical format. Every device attaches to the SCSI bus in a similar manner. Up to 8 or 16 devices can be attached to a single bus. There can be any number of hosts and peripheral devices but there should be at least one host. SCSI uses hand shake signals between devices, SCSI-1, SCSI-2 have the option of parity error checking. Starting with SCSI-U160 (part of SCSI-3) all commands and data are error checked by a CRC32 checksum. The SCSI protocol defines communication from host to host, host to a peripheral device, and peripheral device to a peripheral device. However most peripheral devices are exclusively SCSI targets, incapable of acting as SCSI initiators—unable to initiate SCSI transactions themselves. Therefore peripheral-to-peripheral communications are uncommon, but possible in most SCSI applications. The Symbios Logic 53C810 chip is an example of a PCI host interface that can act as a SCSI target.

Serial Attached SCSI (SAS): - is a computer bus used to move data to and from computer storage devices such as hard drives and tape drives. SAS depends on a point-to-point serial protocol that replaces the parallel SCSI bus technology that first appeared in the mid 1980s in data centers and workstations, and it uses the standard SCSI command set. SAS offers backwards-compatibility with second-generation SATA drives. SATA 3 Gbit/s drives may be connected to SAS backplanes, but SAS drives may not be connected to SATA backplanes.

The T10 technical committee of the International Committee for Information Technology Standards (INCITS) develops and maintains the SAS protocol; the SCSI Trade Association (SCSITA) promotes the technology.

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