SCSI - LQWiki

From LQWiki

SCSI - Small Computer System Interface. Pronounced "scuzzy". It's a specification for a hardware interface for connecting devices such as hard disks and scanners to a computer.

Most PCs have an ATA(IDE) bus instead of SCSI for connecting internal hard disks. SCSI is seen more often in servers, as it tends to be faster and more reliable (though more expensive). Another advantage of SCSI controller is that it requires only one IRQ and can hadle usually at least 7 devices whereas ATA can handle only 2.

Typically, you put a SCSI card in your computer, and then connect internal hard disks with a ribbon cable to some connector on the card. Also, the card will have an external connector which you might also be using simultaneously.

1 SCSI on Linux
2 Types of SCSI
3 See also
4 External links

SCSI on Linux

Linux has SCSI drivers built in the kernel.

On GNU/Linux, the first IDE drive shows up as /dev/hda, whereas the first SCSI drive shows up as /dev/sda. Contrary to ATA, you do not need to configure the devices as master/slave. Each device just needs an unique SCSI-ID. These IDs usually range from 0 to 7. ID 7 is usually reserved for the SCSI controller. Hard disks are usually mounted with smallest SCSI-IDs first, so the computer will first try to boot from the disk with SCSI-ID 0, then from 1 and so on.

Types of SCSI

SCSI-2 is generally same as SCSI-1, but uses a 50-pin connector instead of a 25-pin connector, and supports multiple devices. This is what most people mean when they refer to plain SCSI.

SCSI type	MHz	Bus width (bit)	Data transfer rate (Mbps)	Info
SCSI-1	5	8	5 MBps
Wide SCSI		16		Wider cable (168 cable lines to 68 pins) to support 16-bit transfers. SCSI-2
Fast SCSI	10	8	10 MBps	Doubles the clock rate to support data rates of 10 MBps. SCSI-2
Fast Wide SCSI	10	16	20 MBps	SCSI-2
Ultra SCSI	20	8	20 MBps	SCSI-3
Ultra Wide SCSI	20	16	40 MBps	SCSI-3
Ultra2 SCSI	40	8	40 MBps
Ultra2 Wide SCSI	40	16	80 MBps
Ultra3 Wide SCSI	80	16	80 MBps