Re: [TCLUG:12134] SCSI vs. IDE

[<Nick.T.Reinking@supervalu.com>]

> What's this list's take on this issue?  I've heard from various
>people that even though IDE technically has a higher bulk transfer rate
>with ATA-66, SCSI still outperforms it when you're in a real-world
>environment.
>
> Is this true?

First of all, your "various people" are not correct.  SCSI Ultra-2 has a
maximum transfer rate of 80MB/sec (there's SCSI/160, now, too!), as 
opposed to ATA-66 which is 66MB/sec.  But that's not the important
issue, as the maximum transfer rate is very rarely ever hit.  That being 
the case, let's take a look at some of the other features of SCSI and
features of IDE...


> Also, if you have the typical two onboard IDE controllers, what
>are the interactions?  Can the computer read from the slave CD-ROM drive
>and then quickly write to the master hard-drive on the same controller?
>How about if they're both the master on different controllers?  How much
>does the CPU have to get involved?  Can you queue up reads and writes to
>be handled by the controller via DMA?  Can the queued reads and writes
>be to different devices?  Am I mistaken in thinking that SCSI can do
>this?

IDE is an over-simplified bus.  IIRC, IDE can only address one device on the
bus at the same time.  So, if you had a CD-ROM on one bus, and a hard drive
on the other, then it should be fairly quick.  As a general rule, try to keep 
only
one device on an IDE bus at a time, if you can manage it.  The problem with
that is that most computers with IDE can only handle 4 devices, total.  (That's
2 devices per bus).  The average SCSI controller can handle a total of 15
devices.  Although 4 devices sounds like a lot, my home PC has a total of 5
hard drives, and 2 misc. devices (A DVD-ROM and a CD-RW).  This just couldn't
be handled by IDE.  For the regular computer, IDE should be fine - a DVD and a
hard drive.  My computer tends to keep getting newer hard drives - but I don't
like getting rid of my old hard drives simply because my bus can't handle it.
SCSI can acccess all this devices at the same time, so it's good for copying
files from one devices to the other.  Additionally, it uses a processor on the
controller card, and not the host CPU, so you can do SCSI<-->SCSI copying
without affecting CPU usage at all, really.  IDE has gotten better, in that 
regard,
because of DMA, but it's beginning to creep up again with faster hard drives.
(About 7% of the host CPU's power at 7200RPMs - about 13MB/sec transfer rate).

Additionally, SCSI makes the connecting of external components really easy.
Zip drives, Scanners, etc. work really nice over SCSI, and, as a general rule,
SCSI scanners have far more support under Linux than any other type of scanner.
(like USB or Parallel Port)


> I'm curious because people often ask me to recommend system
>configurations, and I'm wondering if my opinion of SCSI is outdated.
>I'm also wondering how I should recommend they attach their IDE CD-ROM
>and hard drives.

As far as for a recommendation, I'd say IDE to most people.  It's far more cost
effective.  However, SCSI truly *is* better, but you get what you pay for.  
SCSI
costs a lot of money.  Hence, it's good on a workstation, and on servers.  But
for a gaming machine, or a computer for your grandmother, who wants to check
her e-mail, IDE is more than adequate.  Additionally, while not really that 
difficult for a technical person to setup, SCSI does seem extremely difficult 
to
understand for most people, who just want to add another hard drive to their
system.

Well, that's by opinion, take it or leave it!

Nick Reinking