In the beginning, personal computers used cassette tapes to store
programs and data files. The Apple II, Commodore PET, and TRS-80 had
very little memory, so it didn't take too long to load or save
files.
The big breakthrough in that era was Steve Wozniak's incredibly
inexpensive floppy drive controller for the Apple II, which made floppy
drives much more accessible. And the 143 KB capacity combined with
instant access to any sector was simply amazing.
Hard drives started to come into their own after Visicalc, as
personal computers were starting to find a home in the office. At
$4,000 for 5 MB, they were far from a mass market item at first.
Twenty years later we take multi-gigabyte hard drives for granted
and can often find last month's model on sale for under $200 (and
sometimes under $100).
The basic technology hasn't changed much. Precisely machined
platters spin at several thousand RPM. Tiny heads barely avoid touching
the surface, reading or writing data at millions of bits per
second.
What has changed is performance. Twenty years ago a hard drive might
spin at 3000 RPM, hold 5 or 10 MB of information, and fill a
full-height 5-1/4" drive enclosure.
Today's hard drives spin at up to 10,000 RPM, store up to 50 GB, and
come small enough to fit in a laptop. Most drives are 3.5" designs only
about one inch thick.
Standards
Back in the era of the IBM PC-XT, the MFM drive was the norm. Then
came RLL, which managed about 50% more data on the same drive. Soon
came the AT standard, upon which is based the IDE standard seen in most
of today's computers.
Apple took a different route, adopting SCSI in 1986. SCSI drives had
to be more intelligent than AT drives, which required a bit more
computer overhead. Further, SCSI drives could be chained up to 7 per
port. AT drives were maybe two per controller.
Of course, with IBM and MS-DOS setting the standard, over time the
Wintel sector grew to about 90% of the market. Sheer numbers dictate
that all else being equal, market size would make IDE drives less
costly.
But all things are not equal. IDE drives had less intelligence, so
they were less costly to build than SCSI drives - even when the drives
shared almost every component.
In 1994, Apple began using IDE drives in consumer computers. The
Quadra 630 and PowerBook 150 were the first IDE Macs. Today IDE
is the norm, although you can get a SCSI version of the Blue Power Mac
G3.
SCSI vs. IDE
There has been a back-and-forth war between the two standards. Used
to be we saw IDE as slow compared with SCSI. It wasn't necessarily
true, but that was the spin. (Comparison of a Quadra 630's IDE drive and a Quadra 650's SCSI drive under
Speedometer showed virtually identical performance.)
SCSI has several advantages, but speed isn't one of them. Both IDE
and SCSI support data rates far beyond what today's drives can
produce.
For the record, SCSI does allow more devices and does require less
computer overhead. It is a superior technology, but most of the time
IDE is good enough.
Faster, Faster
The typical SCSI connection today is Ultra SCSI, which allows data
throughput up to 40 MB/sec. Ultra DMA/33 IDE falls 25% behind at 33
MB/sec. (FireWire supports up to 50 MB/sec.)
Finding a drive that can move data that quickly is another
story.
There are three ways a hard drive can speed up throughput:
- Spin faster.
- Put more data in each track.
- Use a big cache.
Spin Faster
Although I haven't used 10,000 RPM drives, I've heard they are
incredibly responsive. All things being equal, spinning a drive faster
moves the data past the head more quickly. This results in a linear
increase in speed.
That is, if you could take a 5400 RPM drive capable of 5.4 MB/sec.
and spin it at 7200 RPM, throughput would increase to 7.2 MB/sec. At
10,000 RPM, that would jump to 10 MB/sec.
Increase Data Density
Instead of spinning faster, which also makes the drive run hotter,
you can also write the data bits closer together. Take a drive with 17
sectors per track and increase it to 23 sectors per track for an
immediate 35% performance boost.
Cache as Cache Can
The third way to improve throughput is a bit of a cheat. Put some
memory on the hard drive and let it store data, temporarily buffering
it. If the computer needs the data in the buffer, the drive can provide
that data more quickly from memory than by reading it from the drive
platters.
Sum of the Parts
Today's hard drives are the sum of several parts. While laptop
drives are slower to conserve power, most desktop computers have drives
spinning at 5400 to 10,000 RPM. Data density varies from drive to drive
- and often over the platter within a drive. And buffers have grown as
large as 2 MB in some cases.
You'd think it would be easy to find the fastest drive: get the one
that spins the fastest.
But that's not the way it works.
The current APS catalog lists dozens of drives on page 10. The
fastest write isn't on a 10,000 RPM drive. The APS I 18 GB LVD scores
15.9 MB/sec. while spinning at 7200 RPM. The fastest 10K drive is the
APS I LVD 18 GB LVD 10K, which runs a slightly slower 15.7 MB/sec.
Their both wicked fast IBM mechanisms, but somehow the slower
spinning drive edges out the vastly more expensive 10K drive.
Regardless, look at those maximum sustained throughput rates. Ultra
SCSI and Ultra ATA can handle a lot more throughput, but these drives
(some of today's fastest) can't fill that bandwidth.
It does matter which drive you buy. An economy 5400 RPM drive just
doesn't offer the responsiveness of a 7200 RPM drive, but at 10,000 RPM
you could be paying for performance you'll never see in the real
world.
The same goes for your drive interface. Even the older Wide SCSI
standard was faster than any of these drives, providing for up to 20
MB/sec. throughput.
And many drives are still rated at below 10 MB/sec., which is the
old 8-bit SCSI-2 standard.
Conclusion
Today you can add a ProMAX ATA
card and one or more IDE drives to any PCI Power Mac. Or you can drop
in a SCSI card and one or more SCSI drives.
Both interfaces offer more bandwidth than today's drives, so for
best performance, focus on the drive specifications - especially
sustained read and write speeds. Don't be fooled by 10K drives, since
they may not offer better throughput than 7200 RPM drives, and they
will always cost more.
The only other considerations are price and reliability, and not
necessarily in that order.
The two reasons to choose SCSI over IDE are that you already have
SCSI devices or that you want to chain several external devices to the
computer.
The main reason to choose IDE is bang for the buck. We recently
added 16.8 GB hard drives to a couple Blue G3s at work. Even including
the cost of the ProMAX controller, they were only 60% as expensive as
SCSI drives.
Regardless, you have never been able to buy more megabytes per
dollar than you can today, whether SCSI or IDE.
Further Reading
