Some additional info on Fibre Channel.
From: storage-network.com
See also the white paper by Network Storage Solutions for a Fibre Channel comparison.
With the rise of client networking, the SCSI bus for mass storage is not able to facilitate server-storage connectivity for LAN servers.
Storage Area Network (SAN) is a data communications platform that interconnects both servers and their mass storage at Gigabaud speeds. By combining LAN networking models with the core building blocks of server performance and mass storage capacity, SAN eliminates the bandwidth bottlenecks and scalability limitations imposed by SCSI.
Fibre Channel-Arbitrated Loop (FC-AL) is an open standard that has emerged as the high-speed, serial technology of choice for server-storage connectivity (supported by more than 70 companies)..
FC-AL provides a 2.5 to 10 fold increase in effective data bandwidth over the traditional parallel SCSI storage interface. While the current FC-AL standard for bandwidth is 1 Gigabaud, there are planned enhancements to 2 and 4 Gigabaud. Currently it can provide 200MB/s, protocol support for SCSI and IP, and others, full duplex operation, 126 nodes per loop (additional loops can be added), and a 10 kilometer connection distance (1 Gbaud=100MB/s).
SANs can be built using modular networking devices such as hubs, switches, bridges and routers. Fault tolerant systems can be built using dual loops that provide a redundant path to each storage device in the array in the event that one of the loops is down or is busy.
Competing storage technologies
There are three competing technologies: SCSI, SSA (supported only by IBM), and Fibre Channel.
SCSI is an intelligent, parallel I/O bus on which various peripheral devices and controllers can exchange information. Designed over 15 years ago, SCSI is the oldest peripheral interconnect that is still in widespread use.
The first SCSI standard, now known as SCSI-1, was adopted in 1986, and was originally designed to accommodate up to eight devices at speeds of 5 MB/sec. Since that time, SCSI has been refined and extended numerous times, with the introduction of Fast SCSI (SCSI-2) at 10MB/sec., Fast Wide SCSI (SCSI-2), running at 20MB/sec., and Ultra SCSI (SCSI-3 or Fast-20), which provide data transfer rates of up to 40MB/sec.
One of the main drawbacks of SCSI has always been bus length limitations. Originally limited to six meters, the newer standards, with their faster transfer rates and higher device populations, place even more stringent limitations on bus length, unless expensive differential cabling or extenders are used.
Additionally, the first differential signaling standard, HVD (high-voltage differential), had power requirements that prevent the signal drivers from being integrated into a controller chip. This makes HVD expensive. Low-voltage differential SCSI (LVD), currently in development, features power demands low enough that the signal drivers can be integrated into controller chips, reducing costs and increasing reliability. However, multimode devices (those capable of using either using single-ended or LVD signalling) cannot be mixed on a bus with single-mode devices without losing the advantages of LVD.
Furthermore, even at 40 MB/sec., SCSI is just not fast enough to support modern, multimedia-rich computing applications.
Serial Storage Architecture (SSA)
Limitations for SCSI in terms of bus speed, reliability, cost, and device count are leading toward serial interfaces which are point-to-point interconnections, rather than busses. Serial designs decrease cable
complexity, simplify electrical requirements, and increase reliability.
SSA is a 20MB/s serial interface designed to connect data storage devices, subsystems, servers and workstations. SSA currently is undergoing the approval process as an ANSI standard.
SSA is dual ported and full-duplex, resulting in a maximum aggregate transfer speed of up to 80 MB/sec. SSA connections are carried over thin, shielded, four-wire (two differential pairs) cables, less expensive and more flexible than the typical 50- and 68-conductor SCSI cables.
SSA networks can be constructed using string, loop, or switched topologies. SSA loops can contain up to 126 devices. Devices are hot-pluggable, and some measure of fault tolerance exists because of the redundant connectivity inherent in a loop topology. In contrast to SCSI, which is bus-based and thus has a limit on the total length of all connections, the SSA distance limit is 25 meters per link.
SSA has two protocol levels: the physical level, SSA-PH, and the mapping level. The mapping level allows SCSI commands to be carried over SSA links, which protects existing investments in firmware and software, and simplifies the migration from SCSI to SSA devices.
Fibre Channel Arbitrated Loop
Fibre Channel is an industry-standard, high-speed serial data transfer interface that can be used to connect systems and storage in point-to-point or switched topologies. Fibre Channel Arbitrated Loop (FC-AL), developed with storage connectivity in mind, is a recent enhancement to the standard that supports copper media and loops containing up to 126 devices, or nodes. Like SSA, FC-AL loops are hot-pluggable and tolerant of failures.
The FC standard supports bandwidths of 133 Mb/sec., 266 Mb/sec., 532 Mb/sec., 1.0625 Gb/sec., and 4 Gb/sec. (proposed) at distances of up to ten kilometers. Gigabit Fibre Channel's maximum data rate is 100 MB/sec. (200 MB/sec. full-duplex) after accounting for overhead.
In addition to its strong channel characteristics, Fibre Channel also provides powerful networking capabilities, allowing switches and hubs to enable the interconnection of systems and storage into tightly-knit clusters. These clusters will be capable of providing high levels of performance for file service, database management, or general purpose computing.
The FC standard defines a layered protocol architecture consisting of five layers, the highest defining mappings from other communication protocols onto the FC fabric. Protocols supported include:
Small Computer System Interface (SCSI)
Internet Protocol (IP)
ATM Adaptation Layer for computer data (AAL5)
Link Encapsulation (FC-LE)
IEEE 802.2
Because the details of the FC technology are hidden by the protocol interfaces, the impact on system software is minimal. The lack of system software impact is the reason why an Ethernet would not serve for storage.
All supported protocols can be used simultaneously. For example, an FC-AL loop running IP and SCSI
protocols can be used for both system-to-system and system-to-peripheral communication, sharing a communication path that is as fast as most mainframe backplanes. This capability eliminates the need for separate I/O controllers, dramatically reducing costs, cabling complexity, and board count. (This is especially an advantage over SCSI installations, in which controller proliferation can become a significant problem.). |
