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Fibre
Channel Hubs vs. Switches
In
designing your storage network, one of the most important
architectural hardware decisions is whether to go with
an arbitrated loop (FC-AL) or switched fabric. Arbitrated
loop, with its shared bandwidth and round-robin data
forwarding, was once the only choice.
Relatively
new to the market over the last few years is switched
fabric, which dedicates full bandwidth on each port
and allows simultaneous data transfers to a single node.
Depending on your scaling and performance needs, a simple
hub may suffice. In a larger storage environment, however,
a fibre switch is a necessity. The hubs and switches
in a SAN play a similar role to those in the Ethernet
network.
A
Fibre Channel hub in an arbitrated-loop configuration
is suitable for a workgroup or smaller departmental
environment. Configured with four to 16 ports, hubs
are attractive because they provide a high degree of
interoperability for a low price. As per the Fibre Channel
specifications, hubs support an aggregate bandwidth
of 100 MB per second. Although a hub can support up
to 127 devices, realistically you should restrict the
total to just 30 devices. Because per-port costs of
a switch are significantly higher than those of a hub
(about 3 times the cost per port), a hub is ideally
suited to fan out the core switch ports to the connecting
servers.
Hubs
are limited in their scalability in part because of
the way devices are added to the loop. To become aware
of the other devices in the loop, each device must perform
a LIP (loop initialisation sequencer) when it is first
attached. This action suspends the loop while the entire
population on the loop acquires or verifies the current
port addresses and is assigned an AL_PA (arbitrated
loop physical address). Although the LIPs happen extremely
fast, throughput-sensitive applications, such as tape
backups, are sensitive to hiccups. Also, if one device
on the loop develops errors and floods the SAN with
LIP calls, it could slow the SAN to a crawl or bring
it down entirely. Many vendors have incorporated diagnostics
to isolate the unresponsive port and drop it from the
loop automatically.
One
of the benefits of using hubs is that interoperability
between the different hub manufacturers is not a problem.
The AL_PA process is transparent to the hubs. Therefore,
negotiation between hubs is not required.
The
main disadvantage of hubs is that they severely restrict
scalability and offer no resilience or redundancy to
the network. A Fibre Channel storage network built with
hubs has a single design purpose: high performance.
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