Let's start with the Megahertz Myth today. You've gotta have the
fastest processor possible, right? Sure, we know that an
867 MHz G4 is faster
than a 1.7 GHz Pentium 4. And as long as you have the fastest Mac
around, you're golden, right?
There is always a bottleneck when talking about performance. The
time it takes you to get to work is one example. The bottleneck is that
place on the freeway just before the old bridge where the road goes
from five lanes down to three.
On your G4, is the CPU the bottleneck? At 867 MHz, it's the fastest
part of the computer!
How about the RAM. Obviously, we have level two and level three
cache because the motherboard RAM is so slow. Maybe we should start
using DDR (Double Data Rate) SDRAM in Macs to increase the speed. It
takes the computer 7 nanoseconds or so to access RAM. What a
tremendously long time!
Or maybe access time isn't what we should be looking at. PC133 is 64
bits wide and runs at 133 MHz; that means 1064 MBps throughput. 1.04
gigabytes per second? If you have enough hard disk space to make that
an issue, you may also own BMW.
That gives me an idea - how about the hard drive?
Lets face it, the hard drive is the slowest part of a personal
computer. With access times around 8 milliseconds, it takes the
computer 1000 times longer to access the hard disk than RAM. While the
G4 ships with ATA66 and boasts of 66 MBps throughput, it rarely breaks
35-40 MBps.
Maybe we should ship the next G4 with ATA100 disks? Maybe even
ATA133 disks? There are rumors that the current G4s are capable of
running at those speeds, but Apple has nothing to say on this.
I have a much better idea: SCSI.
Why SCSI? Ultra320 SCSI is almost ready for consumer use. That's a
potential 320 megabytes per second. I realize that today's drives
aren't quite capable of those speeds, but 15,000 rpm drives are
available with access times under 4 milliseconds.
"SCSI is so expensive," you scream. "SCSI is just for digital video
people and graphic designers. Don't you ever read PC World?"
No, I do not read PC World. This is because I think
different(ly).
Once upon a time disk reads and writes were done a lot less than
they are now. I remember getting taught to save my documents every 15
minutes, which seemed like a ludicrously short period of time. Now I
save every three or four minutes. Saving data on floppy disks took
forever, so it wasn't done much. On the Windows side of things,
multitasking wasn't even available until 1995 (at least not in any
meaningful way), so you would usually just save what you were working
on.
Things have changed with the Web.
Every time you look at a Web page, your computer accesses your hard
drive. It saves data in a cache, because your hard drive is faster than
you Web connection. If you use an instant messaging program, they
probably save logs and history files. Every time you get a new email,
your disk is active. With multitasking, sometimes you can be opening
one program while your email is downloading and someone is sending you
an ICQ message. This means a lot of disk access, akin to rush hour at
the aforementioned bridge.
And that's exactly where SCSI beats IDE. SCSI drives are
intelligent, meaning they can process an instruction and let the CPU
get back to work. IDE drives require a lot more CPU time, which isn't a
big problem if you're running one task at a time, but becomes a problem
when you run a multitasking operating system like Mac OS X. The
intelligent SCSI bus even lets devices communicate directly, so one
drive can send data to another drive without tying up the CPU.
Graphic designers and digital video experts have relied on SCSI for
the speed they need, but disk usage in other areas has increased. With
more demands on the drive, there are more reasons than ever to use
SCSI.
The processor in my Blue
G3/450 is good enough for me. Sure, a G4 would be nice, but I would
rather have a few 10,000 rpm Ultra2Wide SCSI drives than an 867 MHz G4
processor.
Andrew W. Hill (a.k.a. Aqua) has been using Macintosh computers since
1987 and maintains that the
Mac
SE is the perfect Macintosh, superior to all - including the
Color Classic. He
is on the verge of being evicted from the family home due to its
infestation of Macs (last count: about 50). Andrew is attempting to pay
his way through college at UC Santa Cruz with freelance Web design and
Mac tech support.
Further Reading
- SCSI & IDE:
Overview and Comparison, Markus Westergren & Mattias Sandgren,
1998 (revised). Good tech overview. Summary: With a single device, SCSI
vs. IDE makes no real difference. However, SCSI is superior when
multiple devices are used because "The ATA devices lacks the
intelligence to perform command queuing like their SCSI
counterparts...."
- IDE vs. SCSI, David
Risley, PC Mechanic, 2001.03.25. "SCSI is a smarter bus than IDE. There
are many steps in the SCSI data transfer. But, on OSes that allow
multitasking, or if you often use several programs at once, the SCSI
drive is a better choice because this extra intelligence of the SCSI
bus is used.
"The performance overhead of SCSI over IDE comes from structure
of the bus, not the drive. The nature of the SCSI bus allows it much
better performance when doing data hungry tasks such as multi-tasking.
The SCSI bus controller is capable of controlling the drives without
any work by the processor."
- IDE or SCSI
Disk, H. Gilbert, Introduction to PC Hardware, 1998 (revised).
"SCSI is worth the extra cost in a Server. EIDE supports two separate
I/O operations to two disks (on the two different interface cables).
SCSI allows all of the disk devices to be active simultaneously."
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