Re: Re:Hardware Compatibility

["Adam Williams" <awilliam@xxxxxxxxxxxxx>]

> I've asked this to a few people on IRC but they thought i was just plain
> stupid and didn't know anything. but what would be the major advantage of
> a SCSI drive at 40mb/sec over a udma/33 drive at 33mb/s. I don't know a
> whole lot about SCSI, so i really don't know.
>

Nah, if you were stupid you wouldn't be asking th question.  Basically there is
more to life than bandwidth.  And NO SCSI or IDE drive is EVER going to sustain
a transfer rate ANYWHERE NEAR those numbers.  That is the maximum "theoretical"
rate which data can move from the drives cache to the system's bus.  Even if a
drive had 1Mb of onboard RAM it would not sustain that quoted rate for even one
second. And that 1Mb of onboard RAM whould have to be full of exactly the data
you wanted. Unlikely.

THe real diffrence between SCSI and IDE is intelligence. IDE doesn't have any.
 IDE handles one transaction at a time, UDMA or not.  DMA simply lets the data
transfer occur without interrupting the CPU.  SCSI devices each have there own
queue of transactions up to 128 deep (though usually less) and the
drives/devices may re-order those transactions if the feel it more efficient to
do so.  Under peice-o-crap barely-multitasking OS like Win95 this is not a real
problem,  IDE performs about the same as SCSI.  Under a preemptive and
multi-user OS like Linux where many programs may be making requests at the
"same time", both read and write the bottle neck of a single transaction-queue
starts to cause a bottleneck.  The data transfers from the devices to the SCSI
adpater occur without any CPU host utilization.  Also a SCSI adapter supports
up to seven or fifteen devices with a max cable length of several feet where as
IDE has two device and about two feet of cable.  Thus you are wasting alot of
bandwidth and lots of IRQ's as your BUS is much faster than any possible
combination of devices that can attach to it (2) and for more device you need
more controller cards which take up system slots and resources like IRQs.  Of
course modern PCI cards can share IRQ channels but then again their is a
performance penalty.

> but i think if i was to switch over to scsi i think i'd want to go with
> scsi wide ultra2 or whatever it's called now. or is it even worth it?
> does a cheap wide ultra2 scsi actually exist?
>
> i don't know why but i get all this scsi stuff confused, scsi, scsi-2, scsi
> ultra wide, ultra2 scsi...
>

You get it confused because it is confusing.  I am confused by Pentium, Pentium
II, Pentium III, Merced, Xeon, Celeron.  It's crazy.  I liked 80286, 80386,
80486, etc... That was nice, it made sense.  Then the 386SX vs. 486SX which
where SX and SX for completely diffrent reasons,  its all down hill from their.