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Ultra SCSI
Ultra SCSI is the next major performance advancement to the Small Computer System Interface (SCSI). Ultra SCSI boosts data transfer rates from the current limit of 10 Mbytes per second for Fast SCSI-2 to 20 Mbytes per second. It also doubles the Fast Wide SCSI-2 data transfer rate from 20 to 40 Mbytes per second. These performance improvements create the bandwidth necessary to support the data intensive applications to be used in the coming generation of servers, workstations and high-end personal computers.
The increased data transfer rates are attributable to the faster cycle times for data transfer and the arbitration of SCSI commands. Host systems and devices using Ultra SCSI will be able to negotiate optimal parameters for speed, width, offset, etc. These improvements in cycle times are primarily fueled by the higher speed of new semiconductor technologies employed in SCSI chipsets.
Easing the migration from the huge investment in other SCSI versions, Ultra SCSI is backward-compatible with previous generations of SCSI and uses the same physical environment. Cables, connectors, and terminators that support SCSI can support Ultra SCSI. More importantly, Ultra SCSI can be integrated without having to modify or change operating systems.
Ultra SCSI White Paper
Introduction
This paper discusses a new feature called Ultra SCSI. SCSI (Small Computer
System Interface) is the best known and most widely used ANSI (American
National Standards Institute) interface. The purpose of this paper is to
show how SCSI has evolved over the years, and inspite of some limitations
existing today, parallel SCSI has many advantages over the other leading
interfaces. Several alternatives are discussed along with the unique advantage
of Ultra SCSI which continues to evolve.
Ultra SCSI and Other Competitive Interfaces
The goal of any peripheral interface is to provide reliable connectivity
and avoid being a bottleneck. Just as the internet has seen dramatic increases
in traffic, the systems which serve the internet are being called on to
deliver unprecedented volumes of data. Internet applications, and others
such as video-on-demand, impact the drive in two ways. First, the drive
itself must be faster, which in turn puts strain on the interface to keep
up. Second, the sheer volume of data being stored in some systems requires
a much larger number of drives than ever before. Some video-on-demand sites
now being set up call for over 1,500 drives in a single subsystem. These
drives are all being connected with the SCSI interface using a large number
of SCSI buses. Clearly this interface has come a long way, but the market
is demanding a giant step forward.
Also, since new generations of high-speed microprocessors are emerging and
with disk density increasing, hard disk drive manufacturers are rolling
out drives with faster read channels and higher sustained data transfer
rates. Improving internal data rates, however, creates a need for faster
external (buffer-to-host) data transfer rates, since using even two of these
super fast drives can saturate a bus, such as the SCSI-2 narrow bus whose
bandwidth permits data transfers of only 10 MB/sec.
These are the main reasons for the feverish activity to develop a serial
SCSI. A technology capable of boosting hard drive data transfer rates into
the 100 MB/sec range eliminating the spectre of interfaces that may otherwise
create a system performance bottleneck.
However, the transition to a new serial drive interface won't be fast, smooth,
or inexpensive. The serial SCSI approaches currently in contention are -
FC-AL (Fibre Channel-Arbitrated Loop) which is a subset of the Fibre Channel
network systems interconnection standard adopted by the ANSI, and SSA (Serial
Storage Architecture). Since adopting any of these interfaces translates
into considerable work and costs for systems manufacturers, many will elect
to postpone their decision rather than lock themselves into a potentially
obsolete technology. And therein lies the catch: the System OEMs need a
high I/O throughput solution now, not two or three years from now.
Fortunately such a solution has arrived in the form of the new Ultra SCSI
interface. An extension of SCSI-2, first proposed by the ANSI XT310 committee
as Fast-20. Ultra SCSI represents an enhancement of the current parallel
SCSI interface standard that doubles SCSI drives' burst data transfer rates
while maintaining backward compatibility.
A Brief History about the Small Computer System Interface
SCSI was created to satisfy the need for a more flexible, faster, command-controlled
interface for hard disk drives and other computer peripherals. Despite the
term "small" in its name, SCSI is large. It's large in use, in
market impact, influence, and unfortunately, in documentation. Early SCSI
was simple, it had a few dozen commands, and the specification could be
tucked in your pocket. The standards effort that began with a 20-page specification
in 1980 has grown to a 600-page extravaganza of technical information.
In 1985, a group of manufacturers approached the X3T9.2 Task Group when
the first SCSI standard was being finalized as an ANSI standard. The group
wanted to increase the mandatory requirements of SCSI and define further
features for direct-access devices. Rather than delay the SCSI standard,
the Task Group formed an ad hoc group to develop a working paper that was
eventually called the Common Command Set (CCS). Many products were designed
to this working paper. The standard provided what were at the time, marvelous
new performance characteristics- it could connect a host with as many as
7 different devices, and data could be moved at an exhilarating 5 MB/sec.
Even before ANSI published the first SCSI standards document in 1986, the
ASC (Accredited Standards Committee) X3T9.2 was hard at work on SCSI-2.
While SCSI-2 was to go beyond the original SCSI standard (now referred to
as SCSI-1), it was to retain a high degree of compatibility with SCSI-1
devices.
No technical rationale can be offered as to where SCSI-1 ended and SCSI-2
began, or as to where SCSI-2 ended and SCSI-3 began. The justification is
much more simple - you have to stop sometime and get a standard printed.
Popular interfaces never stop evolving, adapting, and expanding to meet
more uses than originally envisaged. An ongoing standardization effort began
to extend the capabilities of SCSI-2.
Some of the differences between SCSI-2 and SCSI-1 were that several options
were removed from SCSI-1. Several new low-level requirements in SCSI-2 along
with many options significantly enhancing SCSI. New command sets were added
to SCSI-2 and all command sets were enhanced. Some of the features found
in SCSI-2 include, wide SCSI with data transfer at bus widths of 16 and
32-bits. Fast SCSI achieves 10 MB/sec transfer rate with wider data paths
of 16-, and 32-bits, rising up to 20 MB/sec and even 40 MB/sec. An active
terminator lowering termination boosting system integrity. Several new commands
were added, and diagnostics capabilities extended on the Read/Write buffer
and Read/Write Long commands. Optional messages were added to negotiate
wide transfers to support command queueing which extends up to 256 commands
instead of one command. Sense keys and sense codes were formalized and extended.
Figure 1 SCSI-3 Architecture Roadmap
Accredited Standards Committee X3 has approved several SCSI-3 projects
to enhance and restructure the SCSI-2 standard as shown in Figure 1. These
projects (except fibre channel) are assigned to the X3T9.2 Task Group which
developed this International Standard and the SCSI-1 standard.
SCSI-3 is in fact the sum of a number of separate standards which are defined
by separate groups. It is really a family of standards. SCSI was broken
up from a single document into different layers and command sets. This was
done to allow for different physical transport layers (like fibre channel
and SSA) to be defined, and to allow for smaller "bite-sized"
projects that may get done a little faster. Additional documents for the
Fibre Channel are also meant to be included in the SCSI-3 framework. As
all of this ongoing effort is considerably complex, it may cause changes
in document structure and workgroups.
The New Ultra SCSI Era
Ultra SCSI is the next major performance advancement to the Small Computer
System Interface and is a lower cost alternative to serial SCSI. Ultra SCSI
boosts data transfer rates from the current limit of 10 MB/sec for Fast
SCSI-2 to 20 MB/sec. It also doubles the Fast Wide SCSI-2 data transfer
rate from 20 to 40 MB/sec. These performance improvements create the bandwidth
necessary to support the data intensive applications to be used in the coming
generations of servers, workstations, and high-end personal computers. The
increased data transfer rates are attributed to the faster cycle times for
data transfer and the arbitration of SCSI commands. Host systems and devices
using Ultra SCSI will be able to negotiate optimal parameters for speed,
width, offset, etc. These improvements and cycle times are primarily fueled
by the higher speed of the new semiconductor technologies employed in SCSI
chipsets. Since the internal drivers and the bulk of the drive circuitry
and firmware remain the same, Ultra SCSI amounts to a low-cost, easy-to-integrate
alternative for computer OEMs and system integrators who want to improve
their products' I/O performance. Strategically, Ultra SCSI is the logical
migration from SCSI to accommodate optimal system performance and ever-increasing
demands by users for higher performance. While the infrastructure to support
the Fibre Channel, SSA, and other serial interfaces are being developed,
Ultra SCSI is the cost-effective solution for servers and workstations in
the near term. Over time Ultra SCSI will appear in personal computers as
integration costs decline. By effectively extending the life of the standard
parallel interface SCSI provides a much needed stepping stone to the serial
interface of the future.
Some of the benefits of Ultra SCSI are: easy integration, reusability of
current hardware and firmware, reusability of current test equipment, lower
inventory risk. Due to Ultra SCSI's backward compatibility, it is a low-cost
solution to higher performance. The first Ultra SCSI drives are already
on the market. Drive manufacturers are working closely with host adapter
companies to ensure compatibility across the SCSI bus. The first Ultra SCSI
high capacity drives will feature buffer-to-host data transfer rates of
up to 20 MB/sec for 8-bit Ultra SCSI implementations and up to 40 MB/sec
for the 16-bit Wide Ultra SCSI option. These are double the speed found
in Fast SCSI and Fast Wide SCSI high capacity drives, and follow-on activities
are on the way which should increase these transfer rates even more.
To obtain such performance improvements, and also maintain backward compatibility
with Fast SCSI implementations, and to keep up with the internal disk data
transfer rates, Ultra SCSI doubles the chip's internal clock speed (see
Figure 2). This in turn doubles the millions of bytes that can be transferred
per second. This approach is straightforward but it requires drive manufacturers
to develop innovative solutions to considerable technical challenges in
both digital and analog ASIC design.
To ease the cost of migration from previous SCSI versions which have
involved huge investments, Ultra SCSI is backward-compatible and uses the
same physical environment. Ultra SCSI drives and systems can operate at
the lower speeds of earlier SCSI versions for compatibility with older hardware.
Cables connectors and terminators that support SCSI can support Ultra SCSI.
More importantly, Ultra SCSI can be integrated without having to modify
or change operating systems.
Shorter Cables Equal Clean Signals
Obtaining faster data transfers can pose challenges with analog chip design,
which consists of I/O driver cells or transceivers. The inherent difficulty
with a SCSI interface is that the cable which links the drive and the host
system is a transmission line connection. The fast signal transitions or
slew rates associated with high-performance analog circuit designs often
create transmission-line effects such as ringing, overshoot, and undershoot
which cause signal quality problems, and can interfere with the interface
chip's transceiver performance.
The traditional solution, which is to significantly slow down slew rates,
wouldn't work in this case because it would also prevent the interface chip
from meeting the timing margins needed for Ultra SCSI data transfer speeds
(see Table 1). Instead, Ultra SCSI's developers drew upon their experience
with potential transmission-line effects in designing earlier Fast SCSI
drives: Earlier work had shown that shortening the cable length lessened
the transmission line effects and therefore reduced signal degradation to
acceptable levels. Thus Ultra SCSI drives today require a standard 1.5m
limit on cable length, though work is continuing to increase that length
in the near future. The 1.5m cable length makes the trade-off between slowing
down the skew rates and meeting overall timing requirements easier to manage.
For in-the-box hard drive configurations as well as many outside-the-box
configurations such as desktop system configurations to a CD-ROM, a 1.5m
cable proves more than adequate. For machine room or server environments
in which a large number of devices need to be connected, however, that length
limitation can be restrictive. In these cases the alternative is to use
differential Ultra SCSI drives, which provide greater noise immunity than
the more widely used single-ended SCSI products and can support cable lengths
of up to 25m. In differential SCSI a pair of wires carry each signal that
needs to be sent across the bus. The first pair carries the same type of
signal the single-ended SCSI carries, and the second pair carries its logical
inversion. The receiver takes the differential of the pair thus the name
differential which makes it less susceptible to noise and allows for greater
cable length. Although differential SCSI drives are more expensive than
single-ended SCSI drives their price tags can be easily cost justified for
server and other high-end drive applications.
Table 1 SCSI Bus Timing Values
New proposed devices referred to as bus extenders (which act as repeaters
like those used in communications) let the host be located as much as 6
meters from the first SCSI device. These extenders can convert the 1.5-meter
mini-bus of Ultra SCSI into the stretch limo of buses with an overall length
of 7.5 meters. For many multidrive systems the 1.5-meter limitation on the
back plane connection presents no problem, because the drives are located
in a single box. Bus extenders can be used to create systems in the form
of building blocks, giving designers more flexibility. And the cost is trivial
compared to its benefits: single ended bus extenders are projected to be
priced at (cost) less than $10.
The proliferation of disk drives in servers, RAID, and video-on-demand systems
can stress SCSI's logical connectivity. Under the existing SCSI arbitration
protocol, the logical number of devices that can be addresses is limited
to the number of available data lines: Eight devices including the host
for narrow SCSI, 16 devices for Wide SCSI. But there is a growing need for
more connections. One means of addressing this need is a two-phase arbitration
technique called dual-phase protocol, currently under development in ANSI's
SCSI committee. This addressing scheme is accomplished through delivery
of the higher bits of data of a SCSI-device address in the first phase.
The result of the first phase of addressing determines which devices are
eligible to participate in the second round. The lower bits are then used
to determine the selected device. With this scheme, the number of addressable
devices is equal to the square of the number of data lines, or up to 256
devices with a 16-bit interface, or, for those who demand the ultimate limit,
up to 1,024 devices with a 32-bit parallel interface.
Comparing the Multiple Paths to Higher Performance
There is a great demand in the market today for a giant step forward toward
a very fast, highly efficient hard disk interface. Which interface will
provide a commuter lane for data on the highway where the rush hour never
ends?
The competing serial interfaces, FC-AL and SSA have received a great deal
of attention by the press because they are relatively new, with promises
and dramatic results. In fact, the publicity gained by these serial interfaces
tend to overshadow the impressive gains available in the familiar territory
of parallel SCSI. While the new serial interfaces do offer immediate improvements,
parallel SCSI continues to offer evolutionary growth, and an alternate route
to promising performance.
Throughout the history of innovation we've seen that transitioning from
an existing technology to a new technology has often been less rapid than
anticipated. Invariably, an older technology responds to the same market
pressures that created the new one. This results in enhancements which give
it a whole new life. In fact, in many cases the old technology ends up being
better positioned to respond to the new market demands. For example, in
the magnetic world, thin film heads were envisaged to revolutionize the
industry when they were first introduced six years ago. They indeed boasted
higher signal performance, but simultaneously older technology Ferrite heads
moved forward offering improved MIG (metal-in-gap) designs at lower costs.
Thus, while serial interfaces prove higher performance, some improvements
could make the parallel interface a viable, cost-effective choice.
The SCSI interface has come a long way with Ultra SCSI transfer rates of
up to 40 MB/sec. and increased connectivity for up to 16 devices to be connected
to the bus, as opposed to seven devices when SCSI was first introduced more
than a decade ago. In a year or two transfer rates are expected to move
up to 80 MB/sec with Ultra SCSI, and possibly to 160 MB/sec with the 16-bit
version. With today's high density connectors and direct attachment to the
backplane, this version will actually be easier to configure and more reliable
than the original 5 MB/sec SCSI.
Parallel SCSI has quite a few advantages over the new serial interfaces,
such as the software for I/O in the operating system, drivers and subsystem
logical addressing, firmware, and a fortune in hardware that backward compatibility
can save from the junkyard. Parallel SCSI system design, manufacturing,
support, sources, experience and user familiarity seems a better choice.
There are applications for which even these expanded capabilities don't
suffice. In large scale applications designers resort to multitudes of host
adapters and scary masses of cabling, but the next step solution should
ensure range and load carrying capabilities needed far into the future.
In comparing FC-AL, Ultra SCSI and SSA we find that FC-AL speed leaves other
contenders in the dust. With the turbo charging of dual-porting, FC-AL can
transfer data at 200 MB/sec, with acceleration up to 400 MB/sec not far
off into the future. This bandwidth gives FC-AL a distinct edge when the
goal is to interconnect a large number of drives. Individual drives will
soon offer sustained data rates of over 10 MB/sec, so a 200 MB interface
bandwidth is definitely not overkill in large systems. FC-AL covers distance
spanning more than 30 meters, using copper or coax and up to 10 kilometers
with fiber optic. The requirement for more storage has presented new challenges
such as combining incredible numbers of drives in arrays, and providing
operation without interruption. FC-AL offers viable solutions to these challenges.
Both FC-AL and SSA assure high fault tolerance dual porting. Both parallel
SCSI and FC-AL permit hot plugging, i.e. replacement of drives without disrupting
disk array operation.
Though FC-AL is not expected to appear in products until 1996, ten years
from now it is expected to be the established serial interface. Any practical
implementation of SSA and FC-AL is probably years away, since most users
are comfortable with SCSI. And don't be surprised if a decade from now people
are still debating the decline of parallel SCSI.
Designers and system integrators specifying disk drive interfaces for computers
are faced with a choice between evolutionary and revolutionary technologies.
One path leads to adequate performance improvements with minimal integration
efforts and no additional costs. The other path promises great performance
gains in exchange for additional implementation efforts and costs.
For many designers, the best path is to take full advantage of the performance
improvements offered by the evolutionary technology. Other will forge ahead,
recognizing the advantages of the revolutionary path, and choose to be the
early adopters of the new interface. In the disk drive industry today, the
evolutionary technology is represented by the Ultra SCSI interface, and
the revolutionary technology is represented by the serial interfaces.