Storage Area Networks For Dummies®

Chapter 5: Designing the SAN
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When you connect all these layers together, you create what is known as
a SAN topology — the physical layout of how devices are connected. When
switches are used in the fabric layer, all the interconnected switches are
known as a switched fabric. The three basic SAN topologies for connecting
servers to storage in a SAN are listed in Table 5-1. You can see these topologies
in Figure 5-2.
Table 5-1
Method
Point-to-point
Arbitrated loop
Switched fabric
Basic SAN Topologies
Used for This
Direct-connect a server to storage
Server-to-hub-to-storage
Server-to-switch-to-storage
The different topologies present different challenges. Which one you choose
can affect the performance and resiliency of your SAN. The most common
SAN topology is the switched fabric because it is nonblocking, which means
any device can talk to any other device at the same time (an arrangement
defined as any-to-any). The loop and switched-fabric topologies actually have
sub-topologies, because there are many ways to connect them together. The
way you connect them determines how robust — and costly — your storage
network becomes:
✓ Point-to-point: This topology doesn’t require hubs or switches. It’s the
cheapest to implement but also the most limited in its capabilities. No
surprise there; this network can only consist of two devices.
✓ Loop: This topology uses hubs, and is the next cheapest to build, but
loops don’t scale very well, so they’re best used in small- to mid-size
operations. In a loop, only one device can transmit at a time, so all the
devices have to wait their turn. This limits the effective size of a single
loop to only 127 devices.
✓ Fabric: This is the most common, most scalable — and most expensive —
topology. If you move and store massive amounts of data, this topology is
the way to go. A switched fabric can address millions of devices, so scalability
is not an issue. Also, since the topology is considered nonblocking,
any device can communicate with any other device at any time.
The topology that you choose should be based on your needs and your
budget. The upcoming sections look at each of these major topologies.