Hard Drives- August 27,
1999
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. StandardsBack in the era of the IBM PC-XT, the MLM
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. IDEThere 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, FasterThe 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 FasterAlthough 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 DensityInstead 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 CanThe 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 PartsToday'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 18GB LVD scores 15.9 MB/sec. while spinning
at 7200 RPM. The fastest 10K drive is the APS I LVD 18GB 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. ConclusionToday 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. Recent on Macintosh Online Technical Journal
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