The Fibre Channel interface has the advantage of huge bandwidth.
But it is currently too expensive for most businesses
For the general population it is easy to imagine a data transfer
network as a broad, open, eight-lane freeway, with "blocks" of
different types of data (like video and graphics) moving at high
speed like buses, trucks and cars to various locations all over the
world.
Unfortunately, while much has been made of the concept in the
computing world, there has been little explanation of how it will
actually be implemented. The technical solution used will not only
have to be flexible and inexpensive, it will also require high
performance characteristics to support extremely high data transfer
rates. One solution that addresses those issues head on, but has
not been widely publicised, is a relatively new technology called
Fibre Channel.Fibre Channel is an industry-standard interface
adopted by the American National Standards Institute. It is usually
thought of as a system-to-system or system-to-subsystem
interconnection architecture that uses optical cable between
systems in a point-to-point (or switch) configuration. Certainly,
this is how it was first envisioned, and among the many protocols
defined for it are IPI (Intelligent Peripheral Interface) and IP
(Internet Protocol) which are ideal in those configurations.Fibre
Channel has since evolved to include electronic (non-optical)
implementations and the ability to connect many devices to a host
port - including disk drives - in a relatively low-cost manner.
This addition to the greater set of Fibre Channel specifications is
called Fibre Channel Arbitrated Loop (FC-AL). FC-AL has made it
possible for Fibre Channel to be used as a direct disk attachment
interface, opening whole new levels of I/O performance up to
designers of high-throughput, performance-intensive systems. SCSI-3
(Small Computer Systems Interface-3) has been defined as the disk
protocol, which is also technically refered to as the SCSI-FCP
(Fibre Channel Protocol) for FC-AL.Fibre Channel has been viewed as
too expensive and power-hungry for lower-level functions like
peripheral attachment, raising important questions as to why it
should be used as a peripheral interface. What is Fibre
Channel-Arbitrated Loop? Why use it? How can it be implemented? It
is these questions that we will examine in detail.
What is Fibre
Channel-Arbitrated Loop?The Fibre Channel interface is a loop
architecture as opposed to being a bus-like standard SCSI or IPI.
The Fibre Channel loop can have any combination of hosts and disks
up to a maximum of 126 devices.The loop structure enables the rapid
exchange of data from device to device. A PBC (Port Bypass
Circuit), which is located on the backplane, is the logic that
enables devices to be removed or inserted without disrupting the
operation of the loop. In addition, the PBC logic can take drives
offline or bring them back online by sending a command to any
device to remove it from loop operation or reinstall it onto the
loop.SCSI has become the interface of choice for medium and
large-sized computing systems. At the same time, the computer
industry's experience with SCSI has brought to light the need for
improvements in cable distance, number of variations, command
overhead, array feature support and connectability.
Cable
distance limitationsSCSI comes in several different versions,
but the majority of products shipping today are of the single-ended
variety, primarily because single-ended SCSIs cost less and are
widely available. If a SCSI bus is limited to connecting devices
found within a single cabinet and the interface cable length does
not exceed three metres, it usually is not a problem to use
single-ended SCSI. If the bus must link several cabinets, however,
and in the course of doing so must convert from an unshielded
ribbon cable in one cabinet to a shielded one externally, then back
again to a ribbon cable within the second cabinet, the potential
for SCSI signal problems increases. Differential SCSI solves this
cabling issue, but usually requires that a system have both a
single-ended and differential ports because most non-disk
peripherals use only single-ended SCSI (single-ended SCSI devices
cannot be attached to a differential bus).For many companies
building computers, this creates a problem of logistical
complexity. A systems manufacturer might have to buy two or three
versions of the same drive simply because various system models
require different flavours of the SCSI. Costs can get high when a
company has to account for inventory, sparing, qualification and
testing for multiple versions. Such costs could be avoided if a
single version of the interface were used.SCSI's utility has
improved with increased data rates several times since its
introduction. Unfortunately, only the data rate has improved, which
is the rate at which data is transfered off the disk. The rate at
which SCSI interface information passes over a SCSI bus has not
changed at all. SCSI command overhead is taking up an increasingly
larger proportion of bus time, which makes it difficult to support
multiple drives before the bus is saturated. Often peak transfer
rate is achieved with three drives per each eight-bit bus, because
three drives kept busy will produce so much SCSI protocol traffic
that no additional drives can get any practical bandwidth that
would contribute to the aggregate data rate.Technological advances
mean that magnetic hard disk areal density is increasing at about
60 per cent per year. Since bit density (measured in bits per
inch), one of the two components of areal density, increases at
about 30 per cent per year, data rate automatically increases
proportionately. Disk rotation speeds, which have doubled over the
last five years from 3,600 to 7,200rpm as they continue to climb,
also contribute to higher data rates. In the next few years drives
will be introduced which can sustain transfers in excess of 20Mb/s,
thanks solely to improvements in bit density and rotational speed.
Already there are disk drives in the market which can transfer data
at rates over 10Mb/s. An interface limited to 20Mb/s can only
support one drive at that data rate, making it technically
impractical for future applications.The increased use of SCSI disks
in arrays and fault-tolerant configurations has revealed further
limitations and problems of SCSI, particularly when devices must be
inserted or removed from the interface without disrupting
operations. It has taken considerable ingenuity to get SCSI to run
in the presence of the glitches caused by insertions and
removals.New applications, like video and image processing, have
created a demand for huge increases in storage capacity. Some
capacity requirements are so large that it is difficult to
configure enough SCSI buses to make sufficient drive addresses
available to attach the needed number of drives. Simply increasing
the addressability of SCSI, such as making it possible to have more
than 15 devices per SCSI wide bus, would not be a solution because
more bus bandwidth is needed to support the additional drives.A
disk interface based on the Fibre Channel standard can resolve each
of these problems and provide functionality that has only been
dreamed about up to now. Fibre Channel also has benefits that go
beyond current disk drive interface solutions. Fibre Channel
provides significantly higher bandwidth, superior drive array
features and improved network storage capabilities than
conventional interface technologies.The Fibre Channel loop supports
data rates up to 100Mb/s. Video storage and retrieval,
supercomputer modelling, and image processing are among the
applications growing in popularity that demand this kind of data
rate. Moreover, as file servers are looked upon as replacements for
mainframe computers, they will require ever higher transaction
rates to provide comparable levels of service. Since most Unix and
Windows servers lack the sophisticated I/O channel and controller
structures of mainframe computers, they have not been able to match
the large number of high-performance disk drives enterprise systems
can support. Fibre Channel loops attached to such high-performance
buses as S-Bus, Turbochannel or PCI - all of which run 70Mb/s or
faster -offer I/O configurations that can sustain mainframe-like
I/O rates. Performance estimates suggest that if a system requests
the relatively short I/O transfers typical of business transaction
processing (8K or less), more than 60 drives can be supported
without saturating the loop and bogging down performance. Comparing
the single host adapter to the many channels and controllers
mainframes employ to attach as many drives illustrates the
remarkable economics of Fibre Channel-attached disk storage.Array
controllers have traditionally been constructed with multiple
standard SCSI interfaces for drive attachment, which enables the
controller to supply data and I/O rates equal to several times
those achievable from a single interface. Designing a specific
number of drive interfaces into a given controller, however, has
forced the customer to deal with the parity amortisation,
granularity and controller cost associated with that decision. It
severely limits the designer's choices for configuring the optimal
combination for economy - that is, maximising the number of data
drives per parity drive, granularity - minimising the atomic unit
of capacity per array, and performance. A fully populated
controller is in danger of bogging down under the burden of
supporting the level of I/O activity that several rows of drives
could generate.For example, an array that has six rows amortises
the parity data over five data drives, and usually requires adding
six drives each time any increase in capacity is required. Using
Fibre Channel it is possible for the first time to have more than
enough bandwidth on a single bus to configure an array along a
single interface (the array shown uses two loops to take advantage
of Fibre Channel dual porting for better performance and
reliability, but it could as easily be constructed with one Fibre
Channel loop if full fault tolerance is not called for). Seagate
Fibre Channel drives also have an exclusive XOR logic engine which
allows for easy implementation of RAID 5, thereby avoiding the high
costs associated with traditional RAIDs.Some of the more important
benefits from using Fibre Channel as the basis of a longitudinal
array include:The customer can decide the economy of the array. He
or she can use five, eight, 18 or 24 data drives per parity drive,
with no additional controller cost. The granularity is a single
drive. The customer need not add a whole row of drives as on a
traditional array; he or she can increase the capacity of their
subsystem by just the amount they require. Performance. Because the
controller has been significantly simplified, it is much less
expensive than a multiple drive interface version. The customer can
add controllers - and spread their drives across them - as they
need more performance, but only add drives if they require extra
capacity.
Remote online storageSince the Fibre Channel
interface is part of (and fully compatible with) the Fibre Channel
standard, optical cabling can be used in any part of a subsystem,
excluding the backplane. This makes it possible to have a disk
subsystem quite a distance from the computer system to which it is
attached. Using single-mode fibre optics, online disk storage could
be as far as 10Km away.Fibre Channel is a generic, standard
interface. It has multiple uses and supports many different
protocols, such as SCSI, IPI-3 Disk and Tape Link Encapsulation,
Internet Protocol and ATM. All of these can run on the same Fibre
Channel facility. In fact, some of the first Fibre Channel disk
drive host adapters will support both SCSI and networking protocol.
It makes for a very attractive investment: install a network loop
and get 100Mb/s disk channel for free, or install a 100Mb/s disk
loop and get a 100Mb/s network interface for free. Today's common
discussions about 10Mb/s or 100Mb/s limitations of Ethernet seem
trivial compared with the reality of a network that runs at
mainframe backplane speeds.While there is as yet no software
support for it, Network Attached Storage offers intriguing
possibilities for the future and Fibre Channel makes it
architecturally practical for the first time.
Wide area
networkIn a typical non-fibre channel network the storage
devices would be attached to a file server which would service the
data needs of all other attached systems. The complexity of the
Fibre Channel standard, since it covers so many and because it is
so fast, may lead to the assumption that any implementation would
be extremely expensive. The fact is that several recent
developments have made it very economical.Putting Fibre Channel on
a disk drive will not be significantly more expensive than SCSI
because the interface can be tailored to specifically run the SCSI
FCP protocol. Using the SCSI protocol eliminates much of the
complexity that would come with a comprehensive Fibre Channel
implementation. In fact, the logic associated with Fibre Channel is
only marginally more complex than today's SCSI disk drives.Optical
cable is usually thought to be the most promising vehicle for this
kind of frequency. Recent developments in semiconductor chips have
resulted in inexpensive 1GHz transmitters and receivers. These
chips include all the logic needed to connect the inexpensive CMOS
digital logic that is required for the rest of the Fibre Channel
interface to a 1GHz serial link.The memory speed needed to support
100Mb/s transfers concurrently with 20+ Mb/s disk media transfer
would have been a problem several years ago. The introduction of
Cache DRAMS has made it possible to implement full 100Mb/s
data/cache buffers for the same cost as supporting fast/wide
SCSI.
Compiled by Ajith Ram