For demanding applications,
Fibre Channel (FC) technology has entrenched
itself as the quintessential
storage area network (SAN) interconnect, providing high speed,
high reliability, and inherent security for storage network users.
Fibre Channel technology is a block-based networking approach based
on ANSI standard X3.230-1994 (ISO 14165-1). It specifies the
interconnections and signaling needed to establish a network
"fabric" between servers, switches and storage subsystems such as
disk arrays or tape libraries. FC can carry virtually any kind of
traffic.
Fibre Channel uses optical fiber, coaxial copper or twisted pair
copper cabling to carry SAN data at speeds of 1 Gbps, 2 Gbps, 4
Gbps and (more recently) 10 Gbps. At the same time, latency is kept
very low, minimizing the delay between data requests and
deliveries. For example, the latency across a typical FC switch is
only a few microseconds. It is this combination of high speed and
low latency that makes FC an ideal choice for time-sensitive or
transactional processing environments. These attributes also
support high scalability, allowing more storage systems and servers
to be interconnected. Fibre Channel is also supports a variety of
topologies, and is able to operate between two devices in a simple
point-to-point mode, in an economical arbitrated loop to connect up
to 126 devices, or (most commonly) in a powerful switched fabric
providing simultaneous full-speed connections for many thousands of
devices. Topologies and cable types can easily be mixed in the same
SAN.
Fibre Channel technology denotes four main service "classes" to
meet a variety of enterprise needs. FC Class-1 involves a dedicated
connection running at full bandwidth using delivery confirmations.
FC Class-2 still provides confirmed delivery, but does not use a
dedicated connection. FC Class-3 does not confirm delivery, though
this reduction in overhead can improve apparent performance
slightly. FC Class-4 provides confirmed delivery along with
advanced features such as virtual connections and fractional
bandwidth.
Fibre Channel is regarded as a very reliable SAN technology. The
host bus adapters (HBAs) and switches are
generally quite robust, minimizing the rate of device failures.
The FC SAN fabric allows for multiple connection paths and
redundant connections, so if a hardware fault or cabling issue
arises, a new path can be found and communication can failover
to an alternate connection -- keeping storage and applications
connected (even at reduced performance) until corrective action
can be taken. Alternatively, multiple connections can be
aggregated (or trunked) for even better bandwidth. For example,
two 2 Gbps connections can be aggregated so that they
effectively behave as one 4 Gbps connection. The availability of
multiple or redundant connections enables load balancing where
SAN traffic is analyzed and can be dynamically rerouted from
busy paths (bottlenecks) through less-used paths.
Security is another important attribute of Fibre Channel
technology. A "network" lets multiple devices communicate together.
But for a SAN, it's generally not desirable to allow every server
to recognize or access every LUN on the SAN. In actual practice,
LUNs must be configured so that they are
visible to only certain applications -- configuring security is
a core part of the storage provisioning process. Zoning makes it
possible for devices within a Fibre Channel network to see each
other. By limiting the visibility of end devices, servers
(hosts) can only see and access storage devices that are placed
into the same zone. Once the SAN is zoned, LUNs are masked so
that each host server can only see specific LUNs.
However, there are some recognized disadvantages to FC. Fibre
Channel has been widely criticized for its expense and complexity.
A
specialized HBA card is needed for each
server. Each HBA must then connect to corresponding port on a
Fibre Channel switch -- creating the SAN
"fabric." Every combination of HBA and switch port can cost
thousands of dollars for the storage organization. This is the
primary reason why many organizations connect only large,
high-end storage systems to their SAN. Once LUNs are created in
storage, they must be zoned and masked to ensure that they are
only accessible to the proper servers or applications; often an
onerous and error-prone procedure. These processes add
complexity and costly management overhead to Fibre Channel
SANs.
Check out the entire
iSCSI vs. FC handbook.