A robust RAID solution attached to the new Ultra2 SCSI interface
holds out the promise of better performance and reliability
Applications such as digital video, transaction processing and
data warehousing have placed unprecedented requirements upon
computers and peripheral I/O channels. Distributed data across wide
area and local area networks have created an exponential demand for
servers that can handle small packet sizes and a high number of I/O
requests.
All of the above has pushed server designers to move to RAID
solutions which are truly scaleable in I/O performance. To meet
part of the challenge, a new high bandwidth interface, called
Ultra-2 SCSI (LVD), is entering the market. It allows the server
designer to remove the peripheral channel I/O. Adding to that, new
performance RAID controllers are available which solve the I/O
scalability problem.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 (Small Computer System Interface) is large. It is large
in use, market impact, influence, and unfortunately, 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 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 finalised 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 seven
different devices and data could be moved at an exhilarating
5Mb/s.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 simpler - 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 standardisation effort began to extend the capabilities of
SCSI-2. Several options were removed from SCSI-1 as the document
moved to SCSI-2. New low-level requirements in SCSI-2 along with
many options significantly enhanced SCSI. Advanced command sets
were added to SCSI-2 and command sets were improved. Optional
messages were added to negotiate wide transfers to support command
queuing which extends up to 256 commands instead of one command.
Sense keys and sense codes were formalised and extended.More
importantly, SCSI was broken up from a single document to different
documents which separated out the physical and logical layers. This
was done to allow for different physical transport layers such as
Fibre Channel and SSA to utilise SCSI's rich command set.Ultra-2
SCSI is the next major performance advancement to the Small
Computer System Interface and is a lower cost alternative to serial
SCSI (Fibre Channel. Ultra-2 SCSI boosts data transfer rates from
the current limit of 20Mb/s to 40Mb/s. It also doubles the Fast
Wide SCSI- 2 data transfer rate from 40 to 80Mb/s.These performance
improvements create the bandwidth necessary to support the data
intensive applications to be used in the coming generations of
video, publishing and high-end servers. 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-2 SCSI will be able to negotiate optimal parameters for
speed, width, offset and so on. These improvements and cycle times
are primarily fueled by the higher speed of the new semiconductor
technologies employed in SCSI chipsets.Strategically, Ultra-2 SCSI
is the logical migration from Ultra SCSI to accommodate optimal
system performance and ever-increasing demands by users for higher
performance. While the infrastructure to support 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. By effectively extending the life of the standard parallel
interface, SCSI provides a much-needed stepping stone to the serial
interface of the future.Ultra-2 SCSI is backward compatible and
uses the same physical environment as Ultra SCSI. Ultra-2 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-2 SCSI. More
importantly, Ultra-2 SCSI can be integrated without having to
modify or change operating systems.Finally, new generations of
high-speed multiprocessor servers are emerging. In today's disk
drives areal densities are increasing with hard disk drive
manufacturers rolling out drives with faster I/Os/sec and higher
sustained data transfer rates. The net result has been a growing
demand for higher performance disk servers, which means a
requirement for Ultra-2 SCSI.Drive manufacturers are working
closely with host adaptor companies to ensure compatibility across
the SCSI bus and include: wide SCSI with data transfer at bus
widths of 16 and 32-bits; fast SCSI which achieves 10Mb/s transfer
rate with wider data paths of 16-bits, rising bandwidth up to
20Mb/s; and active termination.Higher I/O requirements at both the
drive side and the host side of the SCSI bus absolutely requires a
leap in peripheral I/O technology. Technologies such as Fibre
Channel have shown bandwidth to accommodate much higher data rates
than that ( up to 1Gb/s. However, the transition to a new serial
drive interface won't be as fast, smooth or
inexpensive.Interoperability and compatibility issues for systems
manufacturers have emerged. This forces many system manufacturers
to postpone their decision rather than lock themselves into a
potentially obsolete technology. And therein lies the catch: System
OEMs need a high I/O peripheral solution now, not two or three
years from now.Fortunately, such a solution has arrived in the form
of the new Ultra-2 SCSI (LVD) interface as proposed by the ANSI
XT310 committee. Ultra-2 SCSI (LVD) represents an enhancement of
the current parallel SCSI interface standard. It has the following
benefits:It doubles SCSI drives' burst data transfer rates to
80Mb/sIt allows up to 15 devices to be connectedSome of the
benefits of Ultra-2 SCSI are easy integration, reusability of
current hardware and firmware, reusability of current test
equipment and lower inventory risk. Due to Ultra-2 SCSI's backward
compatibility, it is a low-cost solution to higher performance. The
first Ultra-2 SCSI drives are already on the market.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 skew 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
is to slow down skew rates. This is because it would also prevent
the interface chip from meeting the timing margins needed for
Ultra-2 SCSI data transfer speeds. Instead, Ultra-2 SCSI's
developers drew upon their experience with potential
transmission-line effects: By using "Low Voltage Differential"
technology the effective length of the Ultra-2 SCSI bus is now 25m
for two devices or 1m for 15 devices. This technology allows older
devices to be connected at a slower bus speed to maintain downward
compatibility.The proliferation of disk drives in servers, RAID and
video systems can stress SCSI's logical connectivity. Under the
existing SCSI arbitration protocol, the logical number of devices
that can be addressed 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. 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.Pushing connectivity into the Storage Area Network,
Ultra-2 SCSI is leading the technology trend. Fibre Channel to
Ultra-2 SCSI RAID bridges are available today. This allows multiple
arrays of drives to be attached to an external controller. In some
configurations, as many as four Ultra-2 SCSI channels are
supported.The embedded RAID subsystem is the engine behind server
performance. It typically includes thousands of lines of RAID
software from which a modern server is operated. It provides many
storage-related functions including:Data I/ORemote Management
Across The NetworkRemote Configuration Across The NetworkSupport
for RAID levelsError AlertsOther Standard RAID featuresTo support
these features while running a large number of I/O processes
requires a more powerful DSP. As I/O requests are processed, the
DSP can service them more effectively. Additionally, as drives are
added to the Ultra-2 SCSI bus, a less powerful RAID controller may
saturate and not scale properly. The net result is that either
another controller is added to scale the servers I/O processing
power or the server reaches a "glass ceiling".With a scaleable RAID
controller, the server starts to saturate as the numbers of drives
are increased. To provide scalability, two key elements are needed,
a modern high speed DSP and a 64 bit PCI interface.
Compiled by
Ajith Ram( 1998 Mylex UK
W.DAD.WP4-T1.190799.DOC I.S. Department 19/08/99 11:40