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In May, I wrote an extensive article on using Ultra Wide SCSI arrays with Macs.
I found when people thought of a RAID, they consider to be a high-end storage solution
not for consumers. I contested this perception and tried to show how performance
applications can benefit from a fast disk array. With the falling prices of SCSI
solutions, high disk performance can for the first time be available to the masses.
The RAID project was grueling and I could not have been happier
to finally put my computer back together. And then Ultra2 came along. For the longest
time, Ultra Wide SCSI had been the high-end standard. Now there is a new technology
in town. Ultra2 is coming to the Mac, and I've spent the past several weeks working
with it. The new flavor of SCSI provides higher bandwidth for data along with finally
bringing some much needed features to SCSI that, hopefully, will save some headaches.
This is Part One in our series of Ultra2 SCSI reviews. Part Two of the series is
a review of Adaptec's 2940U2W Ultra2 SCSI card. Further reviews will follow as we complete them.
Wonderful World of SCSI
First, a quick refresher on SCSI. A SCSI bus is described in
terms of its burst speeds. This is the absolutely fastest rate data can transfer
across the bus. Very rarely however are these speeds actually reached. The typical
Mac
|
| SCSI Term |
Bus Width |
Burst Speed |
| SCSI (I) |
8 bit |
5 MB/sec |
| Fast SCSI (II) |
8 bit |
10 MB/sec |
| Fast Wide SCSI |
16 bit |
20 MB/sec |
| Ultra SCSI |
8 bit |
20 MB/sec |
| Wide Ultra SCSI (III) |
16 bit |
40 MB/sec |
| Ultra2 SCSI (LVD) |
8 bit |
40 MB/sec |
| Wide Ultra2 SCSI (LVD) |
16 bit |
80 MB/sec |
| Wide Ultra3 SCSI (LVD) |
16 bit |
160 MB/sec |
|
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Data Source: Quantum Corp.
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comes with either SCSI-I, Fast SCSI (SCSI-II), or with the newer
PowerMac G3’s, IDE – which is an entirely different ball game. It is important to
note that just because a Maces internal SCSI bus can transfer 10 MB/sec, this does
not mean data will actually travel at this rate. Perhaps if you hooked five hard
drives on the bus and combined all the drives maximum speeds, the full rate of the
bus could be reached. This is a key point – no single drive will ever reach the bus’
maximum burst rate. A number of drives either working together or separately at once
are needed to create some serious bandwidth. (Note: the speed of a hard disk is
a very large factor for transfer rates. I discussed this in the original RAID article.)
It seems 40 MB of data is truly a fantastic amount of information
– or is it? I remember my first Mac had a 20 MB hard drive. Is it possible then to
move twice that in a second? When I first heard Ultra2 Wide would carry 80 MB of
data, I thought that was pretty crazy. However if you look at benchmarks, it is possible
to hit 40 MB of sustained data without much effort. A typical 4 drive Ultra Wide
array can easily reach 30-35 MB/sec of sustained data transfers. So yes, in this
sense it is very possible more than 40 MB of bandwidth could be needed. However after
the real world system and application overhead is taken into consideration, those
benchmarks start to look a little lofty. The Mac OS isn't exactly a lean mean OS
built for speed, and I'm hesitant to say such bandwidth would be considered absolutely
necessary for average use. In other words, I donut consider Ultra2’s 80-MB burst
bandwidth to be its biggest feature. Admittedly, it is nice however to know I've
got 80 MB of bandwidth to play with.
Quantum
suggests a SCSI bus should operate at twice the maximum
speed of a drive set up. If one was to follow this rule of thumb, then a typical
four-drive array, scoring 35 MB/sec with benchmarks, could warrant an card with an
80 MB/sec bus.
Low Voltage Differential Signaling
The next interesting feature of Ultra2 is in the signaling. The rule of thumb is
the faster data moves, the more likely errors will be encountered. With Ultra Wide
SCSI, the cable chain needs to be limited to three meters. Since Ultra2 is moving
data at faster rates, this would mean the SCSI chain would need to be even shorter.
However, Ultra2 utilized something called LVD or Low Voltage Differential signaling.
LVD enables Ultra2 to move data faster along the chain, however it uses differential
signaling, which has lower voltage. The result is LVD is less susceptible to errors.
So, instead of going shorter, Ultra2 cables can go longer. In fact, Ultra2 SCSI can
support a chain of up to 15 meters long. This is a good thing in both adding flexibility
to a SCSI chain and also minimizing errors.
Jumpers for Dummies
Next is something of particular interest to end-users. Ultra2
attempts to take some of the headaches out of SCSI by offering easier termination
and ID selection. With Ultra2, termination is included on the cabling. No longer
will users need to deal with jumpered termination. When using a double ended SCSI
cable however, external termination is needed to "cap" the chain. Also
note that since LVD signaling is used, a special LVD active terminator will be needed.
To clarify, termination is no longer needed on the final drive on the internal chain.
If however the SCSI cable is double-ended (to permit daisy chaining for example)
it must be capped with an external active LVD terminator.
Ultra2 SCSI also features IDs that can be set via software.
Once again, Ultra2 can save users from dealing with jumpers. With the Adaptec software
I tested, if all drives are set to SCSI ID 0, the software package will assign drives
unique IDs. This of course means no more accidental SCSI ID conflicts! It is important
to note that software ID selection and built in cable termination is only available
when using an Ultra2 adapter/cables with Ultra2 hard drives.
The Legacy Continues
Finally what is perhaps the most significant feature of Ultra2 SCSI, is it is backwards
compatible. This means if a user has "legacy" Ultra Wide drives, they can
still use Ultra2 cards. There is of course a catch to this feature. To use an unavoidable
pun, the SCSI chain is only as fast as the weakest drive. In other words,  if an Ultra2 bus has an Ultra2
and an Ultra Wide drive installed, the whole bus will run in Ultra Wide mode. The
same goes with SCSI-II or SCSI-I drives. Adaptec however has created an interesting
solution around this by using SpeedFlex technology. With SpeedFlex,
the Adaptec card offers multiple ports which allows users to connect both Ultra2
and legacy drives to the card without slowing down the Ultra2 drives. On the Adaptec
card, there is an Ultra Wide connector, which can be used when mixing Ultra2 and
Ultra Wide drives. Normally, if the two types of drives are on the same bus, all
drives would run at Ultra Wide speeds. However on the Adaptec card, both drives can
run at their respective speeds. More on this in Part 2 of the Ultra2 review.
With the some of the new features of Ultra2 SCSI under our
belts, lets get to the meat of the technology. To me, Ultra2 SCSI poses two questions.
First, for users new to accelerated SCSI, is it worth investing in Ultra2 SCSI over
Ultra Wide SCSI? Second, is it worth investing into Ultra2 SCSI for users who currently
use Ultra Wide SCSI?
The Need for More Speed
To give some perspective on the various SCSI technologies,
I've provided some benchmarks. All tests were done on an Apple 8500 with a stock
180 MHz 604e, 160 MB of RAM, 256K L2 cache, and 96K disk cache. All drives are fixed
and are formatted with Conley’s SoftRAID drivers. Benchmarks were ran with Remus
benchmarks and Macbench benchmarks. Results with the Remus package represent Remus’
interpretation of actual scores with the Mac OS overhead taken into consideration.
Hardware used for the tests included the Initio Miles Ultra Wide SCSI  card and Adaptec’s PowerDomain
2940U2W Ultra2 SCSI card. Drives used were Quantum Atlas III drives, and Quantum
Atlas II drives for Ultra2 and Ultra Wide tests respectively. SCSI-II tests were
performed used a Quantum Fireball ST drive. (Note: a future installment of the
Ultra2 series will include Seagate Cheetah drives. Also note, these tests were done
on pre-production Quantum Atlas III drives and a pre-production PowerDomain
2940U2W Ultra2 SCSI card. Part 2 of this review will feature shipping hardware.
I am told the finalized products feature a considerably larger improvement in performance.)
Conclusions
As with most new technology, Ultra2 is more expensive. Mac
Ultra2 SCSI cards are just now hitting the market and Ultra2 drives are becoming
more abundant. As expected, Ultra2 SCSI cards are more expensive than regular Ultra
Wide SCSI cards. Currently look to spend between $400-$600 for a Mac Ultra2 SCSI
card. A quick poll of drive prices found Ultra2 drives were typically 10-15% more
than their Ultra Wide counterparts. When buying drives with a SCSI card, bundled
deals can be found. Also look for localized deals and sales on drives. Conspicuous
shoppers can find some great deals.
There is clearly a benefit of Ultra2 over Ultra Wide SCSI.
The difference in speed is significant, however the difference in price leaves some
decisions to be made. There is more than a moderate boost in speed between the two
technologies and Ultra2's bus does double the bandwidth available for transfers.
For the speed fanatics, there is now something faster. For midrange users, new features
such as longer SCSI chains and easier termination and ID selection in itself may
make Ultra2 worth while. Add in the fact an Ultra2 card can support older drives,
a migration to Ultra2 can be done gradually.
There are strong pros to Ultra2 SCSI, with the only set back
being price. Ultra Wide products have become affordable enough for even the most
average Mac users to utilize. Ultra2 offers a step above Ultra Wide in performance
and ease of use, however at a moderately higher price.
In Part 2 of the Ultra2 Review, I will review Adaptec’s PowerDomain
2940U2W Ultra2 SCSI card. I will continue to evaluate the pros and cons of Ultra2
SCSI and Ultra Wide SCSI. Also in the next segment, I will look to see if an Ultra2
card can actually speed up an Ultra Wide system.
I want to thank the people at Adaptec
who have been an enormous help while doing this review. The extra help from the people
at Conley was also much appreciated. I also
want to thank Adaptec for providing equipment for the Ultra2 SCSI testing.
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