There are a variety of disk drives found in computer systems today. Desktop computer systems tend to use ATA based disks whereas servers tend to use Enterprise Class disks such as Fibre Channel or SCSI. Desktop class drives tend to be slower with rotational speeds of around 7200 RPM whereas Enterprise disks are available in 15,000 RPM speeds. Disk drives connect or interface to computers via controllers. The controller may consist of an embedded chip set on a motherboard or, for Enterprise class drives, through a dedicated PCI-based Host Bus Adapter (HBA). The controller communicates with the disk drive via a particular interface.

ATA drives have been available for a number of years on PCs and have evolved to deliver interface transfer speeds of up to 133 MB/s. Many PCs originally housed two ATA channels. Each channel could support two devices configured as Master and Slave. Transfers take place in parallel using across a 16 bit interface to send two bytes at a time. The main advantage of ATA over other interfaces is that of cost. This is largely attributable to economies of scale rather than any other factors. From the point of view of a desktop user, there is little incentive to use anything else.

 

Over the last two years or so, Parallel ATA has been replaced by Serial ATA (SATA) which can reach transfer rates of 3.0 gigabits/second. Many motherboards will still support the legacy parallel ATA (PATA) interface in addition to two or four embedded SATA channels. Serial technologies are actually faster than parallel interfaces and the SATA interface has the distinct advantage that the cabling is simpler and smaller. SATA devices do not share bandwidth and are configured in point to point links. The figure (left) shows a SATA cable on the left compared to a PATA cable on the right. Serial technology allows us to reduce the cable size as there is only one data bit compared to 16 with the PATA cable. In addition the PATA cable has multiple ground wires to ensure noise free data transmission.

SCSI stands for Small Computer Systems Interface and is used in high end Servers. The basic principle behind SCSI is that the intelligence is embedded within the drive and that the drive is capable of accepting a number of high-level commands, which it will execute itself. The original implementations of SCSI sent these commands over eight data lines. These eight data lines are also used for transferring data. With early SCSI implementations there were two types, Single Ended, and Differential. Where the total cable length is less than 6 meters Single Ended is normally used. For lengths between 6 and 25 meters, Differential must be used. Later SCSI variations from Ultra-2 onwards use a single interface known as Low Voltage Differential interface (LVD). Both the adapter and the drive must agree on the same type. High speed devices such as disks today all use wide SCSI, which has 16 data lines and doubles the throughput of eight bit SCSI. A SCSI HBA can support up to 15 devices over a single channel.

Further parallel SCSI development has now been discontinued. Parallel SCSI reached a peak throughput speed of 320 MB/s and has been superseded by Serial Attached SCSI (SAS). The first generation of SAS devices has a peak throughput of 3.0 gigabits per second. SAS features a point-to-point architecture. SAS controllers have the ability to support SATA disks, which means that storage can be tiered to suit the application. For example mission critical devices could be stored on SAS disks whereas transitory data could be stored on the less expensive SATA drives.

Fibre Channel devices are used in high end deployment; they are serial devices and have a throughput of up to 4 gigabits/second. A single loop can accommodate 126 disk devices but this can be extended by the use of Fibre Channel switches. Indeed, this is where Fibre channel devices differentiate themselves from SAS. They are usually deployed as a network of storage devices using optical cables permitting greater speed and cable length. A Fibre Channel network can accommodate some 16 million devices combined into a storage area network (SAN).

The following table summarizes the various technologies discussed in this article:

PATA

Capacity 750GB, Rotational speed (RPM) 7200, Topology Master/Slave, Typical use Desktop

SATA

Capacity 1 TB, Rotational speed (RPM) 7200, Topology Point to Point, Typical use Desktop

SCSI

Capacity 350 GB, Rotational speed (RPM) 10K-15K, Topology Daisy Chain, Typical use Server

SAS

Capacity 500 GB, Rotational speed (RPM) 10K-15K, Topology Point to Point, Typical use Server

Fibre Channel

Capacity 500 GB, Rotational speed (RPM) 10K-15K, Topology Switched/Loop, Typical use Server