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We Still Need SCSI

Andrew W. Hill - 2001.08.31

My Turn is Low End Mac's column for reader-submitted articles. It's your turn to share your thoughts on all things Mac (or iPhone, iPod, etc.) and write for the Mac web. Email your submission to Dan Knight .

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.

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."

Share your perspective on the Mac by emailing with "My Turn" as your subject.

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