>
> I hate to continue to beat this subject into the ground, but exactly what are
> SCSI hard drives considered to the superior to IDE? According to the fellow
> who wrote "Upgrading and Repairing PCs" (who, as a PC technician, I don't
> wholly trust), SCSI hard disks a mostly IDE disks with an additional SCSI
> interface, (ie, the head-disk-assembly is the same, and most of the control
> electronics.) Does SCSI provide additional commands which permit finer
> tuning and/or error prevention/correction than IDE?
>
> It seems to be generally considered that SCSI is superior to IDE, but from
> this guy's writing (excepting for multiple devices on the SCSI bus or really
> fast HDs) SCSI provides no benefit. Pls. someone cursorily clarify.
The SCSI interface was developed by NCR. It is proprietary.
Because if its widespread use an ANSI Standard was developed
to support this interface.
Early disc drive interfaces were all proprietary. When the IBM
PC/XT was developed, the "standard" interface was the "ST-506"
interface, developed by Shutgart. Later, there was the "ESDI"
interface.
As disk drives became larger and faster, new interfaces were
developed to support their increased capability. The ESDI
devices had the capability of dynamically re-mapping bad sectors
so that the drive looked perfect to the operating system. It
did this by keeping a bad-sector table on the drive itself. This
table was an index into the good sectors. A uP on the drive did
the dynamic re-mapping.
Later technology provided spare sectors on each track. This further
increased the speed of the drive. These new drives had a SCSI
interface because this new interface was the highest performance
interface available at the time.
The results were large high-speed disk drives that always looked
perfect to the operating system. These became known as "SCSI"
disk drives simply because they were named for their interface.
As is commonplace in the industry, similar work was done to
reduce the cost of disk drives. The result of a cost reduction
operation was the IDE (Integrated Drive Electronics) drive.
IDE drives combined the AT bus interface electronics with the
drive electronics. This allowed inexpensive drives to be built
with the same performance that one would obtain from a ST-506
interface, without the cost of the drive interface circuitry.
IDE drives are designed to be inexpensive. They don't provide
any performance and reliability enhancing features found in
SCSI drives. IDE drives can have bad sectors which have to
be handled by the Operating System. SCSI drives (if reformatted
occasionally) do not.
Note that with any re-mapping scheme, you can run out of good
sectors to re-map. Low-level formatting of a SCSI Drive will
clean up the drive, leaving only permanent bad-sectors re-mapped.
Hard disks that look the same, with one having a SCSI interface
and the other having an IDE interface, are usually the same
mechanics, but with a different PC Board. The SCSI drive has
a PC Board that contains a uP, some RAM, some program ROM, and
a SCSI interface, in addition to the R/W and servo circuitry.
The IDE drive has a PC board that contains R/W and servo circuitry
plus a cheap port interface.
IDE means "the minimum that will work". It is not a bad/good
thing. A lot of people are not willing to pay more money
for a SCSI drive when their machine us only used on odd Tuesdays.
Many professional Programmers, Engineers, and others who use
PCs continually in their work, insist upon having SCSI disks
in their machines. This does not mean that they are right.
However, I'll not have one of those IDE thingies in my office.
Cheers,
DJ
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Richard B. Johnson
Analogic Corporation
Email : rjohnson@analogic.com, johnson@analogic.com
Penguin : Linux version 2.1.41 on an i586 machine (66.15 BogoMips).
Warning : It's hard to stay on the trailing edge of technology.
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