Editing multitrack digital audio is a demanding task for any
computer, vintage or modern. The storage subsystem gets worked
particularly hard in a way the standard megabytes-per-second
benchmarks don't measure.
Imagine you're a Mac playing back 16 CD-quality audio files
simultaneously: That's 44,100 two-bytes samples for each second and
each track, for a total of about 1.5 MB/s. Sounds easy! Even a
grungy old Quantum ProDrive from a Mac
LC can muster that kind of performance; where's the
challenge?
The difficulty lies in the fact that the job isn't just to read
an 1.5 MB/s linearly from one file. The job is to read 1.5 MB/s
spread across 16 separate files stored in separate places across
the disk surface. Moreover, delivering the data in "spurts" just
isn't acceptable; they must arrive in a regular and timely fashion
to avoid skips in sound playback.
No hard drive that can read 16 files simultaneously and deliver
the contents with perfect regularity has ever been hooked to a
Macintosh. How do we get the sound data to the sound chip at just
the right time then? By the magic of buffering.
Imagine a barrel filled with water with a small hole in the
bottom and someone pouring buckets in from time to time. That's how
the playback buffer smooths out the peaks and troughs in the data
stream from the hard drive. The necessity of using a buffer is
unfortunate, because it introduces a lag in the system - the time
required to fill the buffer - before playback can begin.
More Power!
What's this all got to do with RAID in NuBus Macs?
The Digidesign
AudioMedia II NuBus card still has impressive sound quality
after all these years. With prices so low (would you believe US$10
on eBay?), I want to
build a multitrack audio workstation around one.
I also don't want to wait five or more seconds for playback to
begin. There are two ways achieve this: The first is to use a hard
drive with an extremely fast seek time and a very small, quick
filling buffer. The second is to use a very large buffer and fill
it from a disk (or RAID) with an extremely high transfer speed.
Since the mid-90s, hard drive performance has increased
dramatically. Seek times have come down from about 17ms to 7ms.
That improvement pales in comparison to data transfer speeds: A
fast disk array has improved more than 100 times, from 3.3 MB/s to
460+ MB/s! Thus, for a given load (track count), the second method
is much easier than the first - getting gobs of speed is simpler
than blinding quickness. It was in my quest to fill a huge play
buffer in a reasonable amount of time that I turned to NuBus
RAID.
Doesn't RAID Kill Bugs?
Yes it does, and it also makes your Mac faster. RAID means
Redundant Array of Independent Drives, and there are many types of
RAID with different features. When I talk about RAID in this
article, I mean RAID level 0, which is where two or more drives
share the work of reading and writing files. RAID breaks each file
up into pieces (called Stripe Chunks) and distributes them evenly
to all the disks in the array.
Theoretically, a RAID 0 made of five drives could have a
data-transfer speed five times greater than an individual drive.
Actual results are less stellar, but the improvement is still quite
dramatic - usually.
In my Power Mac 8100 I have a
FWB
SCSI JackHammer, one of the two fastest SCSI interface cards
ever made for NuBus (and a steal nowadays at maybe $20 on eBay).
The other real contender, the ATTO SiliconExpress
IV, would work just as well for our purposes.
I also have a couple of Ultra2Wide SCSI hard drives that are way
faster than anything the JackHammer has ever seen. When I was
disappointed by the speed of filling the playback buffer, I knew
RAID 0 was the best way to go.
"Everybody knows" that if you have two fast disks, striping them
together is "always" faster. Also, "everybody knows" that fancy
third-party hard disk drivers are faster than the stock Apple
drivers, and the fastest ones cost the most.
Ridin', Ropin', and RAIDin'
I benchmarked four different hard disk drivers: ATTO
ExpressPro-Tools, FWB Hard Disk Toolkit 3, SoftRAID 2.2.1, and
Apple's own Drive Setup. All these drivers were running on my Power
Mac 8100 with drives on the FastWide bus of the FWB JackHammer
under Mac OS 7.6. I formatted the drive(s) with test software and
then benchmarked the volumes using ATTO ExpressPro-Tools built-in
benchmarking. Only the FWB and SoftRAID drivers supported RAID, so
Apple and ATTO were tested with single- disk volumes.
Notice anything funny about the graph? In every case, the
fancy RAID 0 configuration was slower than the same
software's standard single-drive setting! Furthermore, all the
drivers come in at about the same speed, with SoftRAID taking a
smallish lead in the Write performance.
Analysis
How this could be is a question I lack the technical know-how to
answer definitively, but I can provide a pretty good hypothesis. 20
MB/s is the theoretical maximum throughput of the JackHammer card,
and real-world performance will always be lower. Thus, for any
particular Mac (and each Mac's NuBus has its own quirks) there will
be a maximum speed that even an infinitely fast drive can't
exceed.
When the speed of an individual drive is much lower than the
maximum the SCSI bus can support, RAID 0 is great for performance.
A little extra overhead chopping the data up for consumption is
rewarded by the drives sharing the load for a huge speed
increase.
That was the situation with vintage drives connected to this
card, and it's also the case for modern drives in modern computers
with SATA and Ultra320 SCSI interfaces.
A vintage Mac with a modern UltraSCSI drive is a completely
different situation. Each of these disks is faster by itself
than the JackHammer is! When you stripe two or more of them
together, the computer chops the data up into little bite-size
pieces. Then each disk devours these little bits of data as fast as
the SCSI bus can supply them.
In this test the JackHammer doesn't seem to be able to exceed
about 13 MB/s. In the RAID configurations, the computer first
manipulates the data and then sends it out to the disk array at 13
MB/s. In the single-drive configs, the computer skips that
processing and just sends the data out at 13 MB/s. If an individual
drive is nearly as fast as or faster than the SCSI bus, using RAID
introduces a bottleneck and reduces performance.
There is one case where this won't happen, even if the drives
are a lot faster than the bus. On Macintoshes with more than one
SCSI bus, such as the 8100, 9150, WGS
95, or any Mac with more than one NuBus SCSI card, RAID 0 might
still help. By placing one drive on each separate SCSI bus and then
striping them, you'll harness the combined speed not of two drives,
but of two busses.
Since used drives are more common than JackHammers, you might
not need one for that 8100 after all!
Putting it All Together
If you're looking for maximum disk throughput in a vintage NuBus
Mac, chances are that just buying and plugging in any fairly modern
68-pin SCSI drive is the way to go.
The other important note is that it doesn't really matter which
formatting software you choose. SoftRAID is faster than the others
by a small margin, but I can't really recommend it for the Macs
we're looking at today. While the Quadras and PowerMacs support
partitions larger than 4 GB under System Software 7.5 and
higher, using SoftRAID limits you to 4 GB on these
machines.
It's also hard to recommend Hard Disk Toolkit. This software's
current version no longer supports vintage Macs, but it is
commercial software, and finding a licensed older copy can be hard,
hard, hard.
Several other drivers like LaCie Silverlining, Micronet Utility,
CharisMac
Anubis, and Casa Blanca Drive7 fit in this "ex-commercial"
category, too. With these eliminated, we're left with only the
freeware contenders.
Apple's Drive Setup utility (and the HD SC Setup that preceded
it) can be patched to
support nearly all SCSI hard drives. You already have a copy; it
comes with the System. It works on PowerPC and 68k-based Macs, and
it can even be used to specify custom mode-pages (if you can puzzle
out the format in ResEdit). Patching Drive Setup to support any
drive isn't as simple as turning a key, though, and those
uncomfortable with ResEdit will be left feeling a little squeamish.
For use on 68k Macs, it's the best bet, so take a deep breath and
try ResEdit.
The downfall of ATTO's ExpressPro-Tools
(registration required) is that it's PowerPC-only. It's freely
available, supports all SCSI drives without patching, and has
easy-to-use Mode Page optimization for Digital Video, Digital
Audio, and PrePress (also good for general use, by the way).
If you are using a NuBus PowerMac, ExpressPro-Tools is the best
choice by far, in my opinion. Check it out!
Until next time, may your disks spin quietly and your Mac scream
with speed. 
Further Reading
