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Ultra
320 White Paper
Introduction
SCSI
celebrates its 20th anniversary with a bang by moving to the seventh
generation of the bus that introduces a maximum data transfer at a
staggering 320 MB/sec. Over the course of the past two decades the protocol
has evolved from an 8-bit, single-ended interface transferring data at 5
MB/sec to a 16-bit, differential interface transferring data at 160 MB/sec.
For the first time the SCSI protocol has been revised in order to reduce the
time spent on processing overhead, resulting in increased performance.
The Three electrical levels of SCSI:
SE=Single Ended; HVD SCSI or Differential SCSI=High voltage differential
SCSI, based on EIA485
LVD SCSI=Low voltage differential SCSI
SCSI’s
commitment to backward compatibility and legacy support are the primary
reasons for its durability as an I/O interface. Throughout SCSI’s 20-year
history, each successive generation of the standard has been backward
compatible with each and every previous generation of SCSI. As a result SCSI
is the industry standard for disk drive connection in virtually all
high-performance servers.
Because of
SCSI’s backward compatibility, migrating to Ultra160 SCSI required minimal
investment. This allowed for a fast smooth transition to Ultra160 SCSI.
Ultra320 SCSI has the same commitment of compatibility and should prove to
be just as easy to use. Ultra320 SCSI is slated to launch in 2001 and will
further enhance SCSI’s legacy in the computer industry.
Key Features
for Ultra320 SCSI
The T10 Technical Committee's SCSI Parallel Interface - 4 (SPI-4) document
currently defines the following features as mandatory for devices supporting
Ultra320:
DT (Double-transition) data transfers:
Double-transition clocking provides a method for the clock signals (ACK and
REQ) to have half the frequency of single-edged clocking at the same data
rate. This is achieved by using both asserting and negating transitions of
ACK and REQ for clocking data. The lower maximum frequency of
double-transition clocking provides more timing margin for ASICs, cables,
motherboard traces, , etc., and reduces EMI issues for system designers.
Free-running clock (FRC):
A free-running clock is used to improve integrity of the clock signal by
removing inter-symbol interference (ISI). ISI is the effect that a
transition on a signal line has on transitions immediately before or after
it on the same line. A pulse (or symbol) will cause a nearby preceding pulse
to shift forward in time, and it will cause a nearby subsequent pulse to
shift backward in time (i.e., a pulse will "interfere" with the placement in
time of its adjacent pulses). By having a clock running at a constant
frequency and a separate lower-speed signal for qualification of data, the
ISI effect is neutralized. The free-running clock is restricted for use with
DT information unit transfers at 320 megabytes per second.
Training pattern:
The training pattern is a pre-determined pattern that is transmitted from
the sender to the receiver at a specified time. The receiver can use
portions of this pattern to perform skew compensation because it knows what
the pattern will be (i.e., exactly when data transitions should occur).
Other portions of this pattern may be used to perform other signal
adjustments such as tuning adaptive active filtering (AAF). The training
pattern may be sent before each data transmission or after some period of
time or an event such as a bus reset.
Skew compensation of data signals relative to the clock signal:
Skew is the difference in time from when one transition launched by a sender
arrives a given point (e.g., a recipient's connector) to when a second
transition launched by the sender arrives at the same point. The arrival
time difference is caused by several factors including differences in length
and electrical characteristics of the two signal paths. If a data transition
is skewed so much relative to the clock that it falls outside of the
qualifying clock window, the device will not accurately detect data. One of
the largest losses in the error budget for Ultra320 SCSI is skew. With the
training pattern specified, an Ultra320 SCSI device can establish skew
compensation simultaneously for each of the received transitions on the data
lines so that they occur at the correct time relative to the clock. Once
established, this compensation is used for all subsequent transfers until
the next training pattern sequence is initiated.
CRC (Cyclic redundancy check):
CRC is an algorithm that provides improved data protection for the parallel
SCSI bus. A sending device uses the algorithm to generate check bytes for
transferred information. These check bytes are transmitted immediately
following the information. The recipient uses the same algorithm to
calculate check bytes from the received information and compares the result
to the received check bytes. If the two sets of check bytes match, the
information is correct. CRC is defined for use only with DT transfers.
CRC provides extra data protection for marginal cable plants, external
devices, and is one of the best ways to assure data protection during hot
plugging. SCSI CRC dramatically reduces undetected error rates by using the
same proven algorithm that is used by the FDDI, Ethernet, and Fibre Channel
interfaces.
The SCSI CRC detects:
· All single bit errors,
· All double bit errors,
· All odd number of errors
· All burst errors up to 32-bits long, and
· Has an error rate of approximately one in 2 ´ 1032 for random error
patterns
Domain Validation (also known as "Physical layer integrity checking"):
Simple domain validation describes how an initiator can use the INQUIRY
command to query targets to determine their capabilities (e.g., maximum
transfer rate), the system configuration (e.g., the width of the bus), basic
functionality of the system components, and how the initiator can use the
READ and WRITE BUFFER commands to send and receive known data patterns from
the targets for rudimentary data integrity checking.
Information unit transfers (or "IU transfers", also known as "packetized
transfers"):
IU transfers provide a protocol to increase overall system performance. Some
of the elements of the protocol include:
· A method to encapsulate non-data information (like commands sent from the
initiator to the target and status sent from the target to the initiator)
into packets and transfer those packets at the maximum negotiated data rate
of up to 320 megabytes per second - as opposed to those same transfers
occurring in asynchronous mode at five megabytes per second or less;
· A method to transfer packets for a number of I/O processes without an
intervening physical disconnection (e.g., an initiator could send several
packets each containing a queued command to the target during a single
physical connection);
· A method to minimize overhead by eliminating several bus phase changes per
I/O process, for example: a typical WRITE operation using normal data group
transfers would require ARBITRATION, SELECTION, COMMAND, DATA OUT, STATUS,
and MESSAGE IN phases. The same WRITE operation using IU transfers would
only require ARBITRATION, SELECTION, DATA OUT, and DATA IN phases. The
command and data would be transferred during the DATA OUT phase, and the
STATUS and COMMAND COMPLETE message information would be transferred during
the DATA IN phase, all at the maximum transfer rate.
Transmitter pre-compensation with cutback:
Transmitter pre-compensation with cutback is an open-loop method of
compensating for some of the signal loss that is most severe on the first
part of a signal's transition. The transmitting device boosts the amplitude
of the first part of the transition, or cuts back the signal for the
remainder of the transition. This provides additional signal amplitude where
it is most needed and then decreases the amplitude to decrease the negative
effects of cross-talk and reflections.
Backward compatibility:
Backward compatibility means that a device supporting a new feature set can
be used with legacy devices that only support transfer rates and protocols
previously defined for the SCSI interface. Examples include: the ability for
transceivers to operate in "single-ended" (SE) mode (as opposed to
"low-voltage differential", or LVD, mode required by the higher transfer
rates), the ability to tolerate five volt single-ended signaling from other
devices, and the ability to function properly with the current cable plant
specifications (i.e., a 25-meter cable in a point-to-point configuration
with only two devices attached or a 12-meter cable with up to 16 devices
attached).
Optional Features for Ultra320 SCSI
The SPI-4 document has defined the following optional features for devices
supporting Ultra320:
AAF (Adaptive Active Filter, also known as "receiver equalization with
filtering"):
AAF is a closed-loop method of improving received signal quality by
amplifying the fundamental frequency of the signal while filtering noise and
other undesirable components. Devices implementing AAF establish the gain of
its amplifiers by setting the amplitude of the high frequency portion of the
training pattern to be the same as the low frequency portion at the
beginning of the training pattern. Using the training pattern to perform
this adjustment of signal amplitude provides for an inherent closed-loop
system that can adjust signal quality for different cable plants and changes
in system conditions (e.g., when a new device is added to a system causing
the electrical characteristics of the cable plant to change). AAF settings
may be adjusted as often as necessary because either the initiator or target
may initiate at the training pattern sequence. A receiver may disable
transmitter pre-compensation in a transmitter as AAF performs better in the
configuration.
QAS (Quick Arbitration and Selection): QAS provides increased overall
system performance by allowing arbitration to occur without incurring the
overhead of intervening BUS FREE phases, saving up to microseconds per
operation. This is significant when compared to the 1.6 microseconds it
takes to transfer each 512-byte sector of data at 320 megabytes per second.
Rather than waiting for a BUS FREE phase, a target may initiate arbitration
by issuing a QAS REQUEST message. The devices on the bus may "snoop" the
message and participate in the arbitration. QAS can only be enabled if
information unit transfers are enabled.
SCSI bus fairness (fairness):
Fairness prevents a device from "hogging" the bus by guaranteeing that all
devices have an opportunity to arbitrate. Fairness must be enabled when QAS
is enabled, as "hogging" could potentially be more of an issue with the QAS
protocol. The standard method of arbitration for parallel SCSI is that the
highest SCSI ID on the bus always wins arbitration. For fairness
arbitration, the SCSI devices monitor arbitration attempts in a fairness
register. If a device has won an arbitration during which other devices with
lower SCSI IDs had also participated, the device checks its fairness
register during the next arbitration. If a device with a lower SCSI ID had
participated in the previous arbitration, lost the arbitration, and is
participating in the current arbitration, the device that had won the
previous arbitration will drop out, allowing the device with the lower SCSI
ID to win the arbitration. The device with the higher SCSI ID will not
participate in an arbitration until all devices with lower SCSI IDs have had
an opportunity to win an arbitration.
Flow Control:
Flow control provides a method during a data streaming operation for a
target to give an indication to an initiator that the current SPI data
stream information unit is the last of the current stream. This early
warning provides a way for the initiator to more effectively manage other
resources.
AIP (Asynchronous Information Protection):
AIP provides an enhanced error detection method for the COMMAND, MESSAGE,
and STATUS asynchronous transfer phases by transferring error detection
information (a BCH Hamming code) on the upper eight data bits of the data
bus simultaneous with the information transfer. The recipient uses the same
Hamming algorithm to check the received information. If the result of the
calculation is correct, the information transferred is correct.
Applications For Ultra320 SCSI
With the
acceleration of microprocessor performance, bottlenecks in the I/O channel
continue to be a cause for concern. SCSI continues as the workhorse
technology that addresses this problem. With a transfer rate of 320 MB/sec,
Ultra320 SCSI is the next step in the evolution. With PCI-X delivering bus
rates up to 1066 MB/sec, high-performance SCSI I/O allows for greater speeds
across the entire PC bus.
As computer
systems increase in capability, new applications evolve to take advantage of
the available power and features. For example, desktop publishing,
scientific visualization, video and audio editing, digital broadcasting and
other data-hungry applications continue to push the I/O bandwidth and
require a more advanced interface to handle increased data transfer.
In addition
to the increased speed of 320 MB/sec, the new technology that reduces
overhead will benefit real transaction process applications such as
data-mining, material requirements planning (MRP), and other database
programs. Random access applications such as these can involve searching for
data on many different disk drives on the server. Technology such as QAS
will benefit these applications by reducing the overhead of control release
from one device to another on the SCSI bus.
To keep up
with the multiple data streams that today’s processors can accept and
generate, high-performance RAID arrays are required. These large disk farms
with RAID configurations can also benefit from Ultra320 SCSI’s high
bandwidth. For example, high-end workstations have applications where they
must merge several video and audio clips from different channels and disk
drives. A high-performance RAID array can have between eight and fifteen
disk drives attached to a single channel. Include a dual channel controller
and the total array can be up to 30 disk drives, which makes SCSI a natural
choice. Finally, Ultra320 SCSI’s transfer rate of 640 MB/sec across both
channels will insure that there is adequate bandwidth to provide maximum
performance.
New
streaming video and audio editing applications have taken advantage of the
accelerated performance of I/O. Ultra320 SCSI will provide the bandwidth to
manage tomorrow’s increasingly rich collection of dynamic media. Media
creators require performance that allows them to work faster and more
efficiently. Ultra320 SCSI provides the speed, capacity, scalability and
reliability that these I/O hungry applications require.
As demand
for external storage in the SAN environments continues to grow, Ultra320
SCSI insures that the technology is there to allow integrators to take full
advantage of their existing installed base and not effect the performance of
Ultra320 SCSI. With SAN, customers have an extensive fabric connecting to
several SCSI drive boxes throughout their company. Ultra320 SCSI maintains
compatibility with existing low voltage differential (LVD) SCSI technology
and allows customers to mix new and old technologies without interruption
Conclusions
SCSI enters
its 20-year anniversary by ushering in a new technology. Ultra320 SCSI is
sure to add to the existing legacy of past SCSI technologies. SCSI has come
a long way from its original 5MB/sec transfer rate. At 320 MB/sec, Ultra320
SCSI is only the latest in SCSI evolution. As technology continues to move
into the 21st century, the industry can continue to look forward to new and
faster SCSI technology. Ultra640 is already in development.
With new
technologies such as packetized SCSI, QAS, training and pre-comp, SCSI will
continue to deliver performance safely and reliably for generations to come.
As performance continues to grow, so will the applications that can take
full advantage of greater I/O performance. PCI-X accelerates performance
across the host bus to 1066 MB/sec and Ultra320 SCSI is there to take full
advantage of this available bandwidth.
And as
always, SCSI maintains its backward compatibility allowing customers to
protect their investment while concurrently giving them the ability to grow
as their needs increase. No other I/O technology can provide these
advantages. SCSI continues to increase its performance, features,
enhancements and market share. Ultra320 SCSI is the newest example of SCSI’s
continued commitment to providing the industry with the I/O bandwidth
necessary for an increasing number of performance hungry applications. SCSI
will continue to evolve and with Ultra640 SCSI already on the roadmap, it
will be impossible to replace.
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External RAID subsystem