Re: Ultra DMA IDE vs. SCSI 2 or 3

[Steven Pritchard <steve@silug.org>]

Richard H. Fifarek said:
> 	How so?  If you read the numbers, Mode 4 IDE drives go at 16MB/s,
> whereas SCSI usually top out at 10MB/s, right?  If you want the higher
> performance out of the SCSI, don't you have to use mirroring/striping?

"Fast SCSI-2", an *old* technology, tops out at 10MB/sec.  There's a
20MB/sec SCSI standard, but most newer SCSI drives (usually called
"ultra wide") are 40MB/sec.

FWIW, all this standards confusion in SCSI is because SCSI-2 is dated,
and SCSI-3 still isn't out, or at least implemented by anyone.  So
everything SCSI you see now is SCSI-2 with some SCSI-3 extensions.

> > I'd surely try to go with the fastest hardware to prevent hardware and 
> > network failures down the road.  If your hardware can't keep up with your 
> > load, then you will have to upgrade later and end up spending more $$ 
> > than you started with.
> 
> 	Exactly.  The problem that we are running into is that everyone
> sings the praises of SCSI and swears that it is the only way to go, but
> there are a few of us that aren't so sure (myself being one of them).
> I was just hoping to get some opinions and tidbits of info that I should
> take into account.  Personal experiences, general rules to follow, etc.

The biggest reason that SCSI is the way to go is that it requires
*much* less CPU overhead to operate.  (I would paste my canonical
demonstration, but it appears my CPU is so much faster than it used to
be that my old IDE drives aren't much overhead...)  In my experience,
IDE seems to take about twice as much CPU to operate.  This may not
seem like such a big deal on a modern machine, but think about the
worst possible situation a server can be in...  Heavy disk usage while
swapping heavily.  (Imagine a kernel compile, or several if the
machine is really fast.  :)  Imagine that server thrashing away.  Now,
think what the difference in responsiveness would be with IDE vs.
SCSI, if SCSI uses only half the CPU to operate as IDE, and is
generally faster.  (If I'm not mistaken, SCSI won't tie up the bus as
much either, but I could be mistaken about that...)  Of course, the
server with SCSI disks will be much more responsive, and much less
likely to thrash itself to death.

If you go SCSI, don't for a minute think that you have to spend a lot
of money.  I don't know why, but prices in SCSI drives are insane...
Essentially the same drive can cost twice as much from one
manufacturer to another, or from one distributor to another.  Look at
the specs, and shop around.  More importantly, note that all SCSI
controllers are most definitely *not* created equal.  The Adaptec
2940s are expensive as hell, and don't work worth a damn on Linux.
The NCR/Symbios 53c8xx work on every platform, are every bit as fast
as the 2940s (possibly faster), and are usually less than half the
price.  I've had a generic NCR 53c810 and a Diamond Fireport 40
(Symbios 53c875), and have been quite happy with both.  (The alpha
also has the 53c810 on-board, which works quite nicely.)

Now, the fact remains that IDE is cheaper, and does offer reasonable
performance these days.  (Of course, you get into a real chicken & egg
thing with IDE prices vs. SCSI prices...  Since more IDE gets used,
it's cheaper.  Since IDE is cheaper, more people use it.  Repeat.  But
that's beside the point.  :)  If cheap is more important than any
other consideration, then by all means go IDE.  While you are at it,
be sure to get a K6.  They rock, and they're cheap.  (Hell, get one
anyway.  You'll be happy.)

If you go IDE, and you regret the decision later, at least you won't
have spent much, and you can always upgrade to SCSI...

Steve
-- 
steve@silug.org           | Linux Users of Central Illinois
(217)698-1694             | Meetings the 4th Tuesday of every month
Steven Pritchard          | http://www.luci.org/ for more info

--
To unsubscribe, send email to majordomo@silug.org with
"unsubscribe silug-discuss" in the body.