Apple Xsan 2 (Third Edition) - Manual (Page 15)

Chapter 1 Overview of Xsan 15
Shared SAN Volumes
A user or application on a client computer accesses shared SAN storage just like they
would a local volume. Xsan volumes are logical disks made up of pools of RAID arrays.
The elements you combine to create an Xsan volume are described in “How Xsan
Storage Is Organized” on page 16.
Metadata Controllers
When you set up an Xsan SAN, you assign at least one computer to act as the
metadata controller. The controller manages volume metadata, maintains a file system
journal, and controls concurrent access to files. Metadata includes such information as
where files are stored and what portions of available storage are allocated to new files.
To guarantee volume availability, a SAN should include more than one metadata
controller, as shown in the illustration on page 14. If the primary controller fails, the
standby controller takes over.
Clients
The computers that users or applications use to access SAN volumes are called clients.
Clients exchange metadata with controllers over the private Ethernet network but use
Fibre Channel to send and retrieve file data to and from the RAID systems that provide
storage for the volumes.
Network Connections
Xsan uses the following independent networks to connect storage devices, metadata
controllers, and client computers: a Fibre Channel network and two Ethernet networks.
Fibre Channel
Xsan moves data between clients and SAN volumes over high-speed Fibre Channel
connections. Controllers also use a Fibre Channel connection to move metadata to and
from the volume.
Xsan can take advantage of multiple Fibre Channel connections between clients and
storage. Xsan can alternate between connections for each read and write, or it can
assign each RAID array in a volume to a connection when the volume is mounted.
Ethernet
Xsan controllers and clients exchange file system metadata over a separate, private
Ethernet network. (Controllers use Fibre Channel to read and write metadata on a
volume.)
To prevent Internet or intranet traffic from interfering with metadata communications,
set up separate public (Internet) and private (metadata) Ethernet networks, as shown
in the illustration on page 14.