[NTLUG:Discuss] SCSI drives

[Steve Baker]

Vaidya, Harshal (Cognizant) wrote:

>   Can anybody explain me what is the difference between a normal Hard Drive
> and SCSI drive.


SHORT ANSWER: They use a different bus connection.  You can't just plug a
               SCSI drive into your motherboard.

LONG ANSWER:

'Normal' hard drives connect via the IDE interface which only allows (I think)
two drives per interface.  Most PC's have two interfaces - limiting you to four
drives total.

SCSI drives use the SCSI (Small Computer System Interconnect) interface which
allows a wide variety of devices to be connected to the bus (not just disk
drives) and can have up to (IIRC) sixteen peripherals connected to it.  The
SCSI standard is rather overkill for disk drives because it allows for all
sorts of sophisticated behaviors that disk drives don't need.  My scanner
is interfaced via SCSI for example.

Historically, SCSI drives have been more expensive and faster than IDE drives.

However, with the more recent varients of IDE and SCSI, I've rather lost track
of which is better these days.

Additionally, most motherboards lack a SCSI bus connection - so you'll probably
have to go and buy a SCSI adaptor.

SCSI is also better suited to external drives than IDE because it's electrical
specification allows for much longer cables than IDE - and there is a standard
connector that's better suited to external ports than the flimsy IDE connector.
External SCSI devices are 'daisy-chained' - so each device has an input and an
output connector - the last one in the chain has to have a 'terminator' plugged
into it's output port.

Some software (eg CD-burners) *REQUIRE* a SCSI interface - Linux has a SCSI
emulator that can make IDE devices appear to be SCSI devices even though
they are not connected that way.

CONCLUSIONS:

* For most uses, get an IDE drive and just plug it in and go.

* If you need more than 4 drives in your PC (bearing in mind that
   your DVD and CD-ROM drives are probably on the IDE bus) - get SCSI.

* If you really, deeply care about disk performance then you'll
   want to do a TON more research - and you may well find that
   SCSI is still the better interface.

* Don't buy a SCSI drive without bearing in mind the need for s
   SCSI adaptor card in your PC.

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