Apple Xsan 1.0 - Manual (Page 9)

Chapter 1 Introduction to Xsan 9
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 10.
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 can add more than one controller to a SAN, as shown in the
illustration on page 8. If the primary controller fails, the standby controller takes over.
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.
A SAN client might be an individual user’s Mac OS X computer or a server running
Mac OS X Server. 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.
SAN Connections
Xsan uses independent networks to connect storage devices, metadata controllers, and
client computers: a Fibre Channel network and one or two Ethernet networks.
User Data Over Fibre Channel
User data is transferred over high-speed Fibre Channel connections.
Metadata Over Ethernet
To eliminate unnecessary traffic on the Fibre Channel connections, controllers and
clients use an Ethernet network to exchange file system metadata. The Xsan Admin
application also uses the Ethernet connection to let you manage the SAN. To prevent
other network traffic from interfering with metadata transfers, you can use two
separate Ethernet networks; one private for the SAN, and the other public.
Fibre Channel Multipathing
Xsan can take advantage of multiple Fibre Channel connections between clients and
storage. Xsan can alternate between connections for each read and write, or assign
each LUN in a volume to one of the connections when the volume is mounted.
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