Apple Xsan 1.4 - Manual (Page 21)

Chapter 1 Overview of Xsan 21
Shared SAN Volumes
Users and applications see shared SAN storage as local volumes. Xsan volumes are
logical disks made up of groups of RAID arrays. The elements you combine to create an
Xsan volume are described under “How Xsan Storage Is Organized” on page 23.
Controllers and Clients
When you add a computer to an Xsan SAN, you specify whether it will play the role of
client, controller, or both.
Controllers
When you set up an Xsan SAN, you assign at least one computer to act as the
controller. The controller manages the SAN volume metadata, maintains a file system
journal, and controls concurrent access to files. Metadata includes such information as
where files are actually stored and what portions of available storage are allocated to
new files.
For high availability, you should add more than one controller to a SAN, as shown in
the illustration on page 20. If the primary controller fails, the standby controller takes
over. Though not recommended for best performance, controllers can also act as
clients, so you can use a standby controller as a working client while the primary
controller is operational.
Clients
The computers that users or applications use to access SAN volumes are called clients.
Clients communicate with controllers over the Ethernet network but use Fibre Channel
to send and retrieve file data to and from the RAID systems that provide storage for the
volumes.