Storage Area Networks For Dummies®

Chapter 5: Designing the SAN
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Actually, a point-to-point topology is a way of using a fast, direct, optical
Fibre Channel connection between the server to external storage, as shown in
Figure 5-4. That keeps the server’s hard drives separate from the server itself,
so you can do things like upgrade the server without replacing the disks.
Figure 5-4:
External
storage
using a
point-topoint
SAN.
Disk
storage
Disks
Cable
Server
Using an external storage array connected via the point-to-point topology
enables sharing that storage among more than one server. This is not possible
with DAS (direct attached storage) because a server usually can’t connect
to the hard drives inside another server.
The number of servers you can connect to a storage array by using a pointto-point
topology depends on the number of Fibre Channel ports your storage
array has available.
The larger, more expensive storage arrays can come with dozens to hundreds
of Fibre Channel ports. The smaller, more modular arrays usually come
with between four and eight Fibre Channel ports.
Point-to-point topology requires a dedicated port on the storage array for
each server connected to it. For example, if you have a modular array with
four ports, you can share that storage among four servers. But if you have one
of the large arrays, you can share that array among 64 or more servers using
a direct connection! This is all possible without buying a single hub or switch.
All you need is a Fibre Channel HBA in each server and a cable from each
server to the storage port on the array.
Point-to-point is a good topology choice for people on a tight budget with only
a few servers that use inexpensive storage, or for a system with a few very
large servers (such as a mainframe) with one big, expensive storage array.
You don’t need any expensive switches or hubs to make this work, which
saves on the up-front costs.