Apple Xsan 1.1 - Manual (Page 8)

8 Chapter 1 General Information About Upgrading
If Volume Availability During the Upgrade Is Not Important
If your SAN volumes don’t need to be available during the upgrade, you can stop the
volumes and then perform the upgrade as you would any software upgrade, without
relying on controller failover. However, if you are upgrading to Mac OS X or Mac OS X
Server v10.4, be sure that you choose to perform an update installation, not a clean
installation. Otherwise, you’ll lose your volume and SAN configuration files.
About Primary and Standby Controllers
Each Xsan volume is hosted by a metadata controller, called the volume’s “primary
controller.” To ensure availability of the volume and to protect against data loss, you
usually set up at least one other computer, called a “standby controller,” to act as a
backup. If a volume’s primary controller or the file system processes running on it
become unresponsive, the standby controller takes control of the volume during
controller failover.
On a controller that is hosting more than one volume, it’s possible for a single volume
to fail over to a standby controller while other volumes hosted by the controller
continue unaffected. The volume that failed over now has a different primary controller
than the other volumes. So, in a SAN with more than one volume, each volume can,
through the process of failover, end up on a different primary controller.
The instructions in this guide take advantage of failover to maintain volume availability
without your having to know which controller is hosting a volume. All you need to be
sure of is that there is another controller ready to host volumes that are currently
hosted on the controller you are upgrading.
If You Don’t Have a Standby Controller
If you have set up your SAN with a single metadata controller, you can do one of the
following during the upgrade:
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Unmount and stop all of your SAN volumes for the duration of the upgrade.
(The volumes will be unavailable during the upgrade.)
 Temporarily promote a client computer to the role of controller during the upgrade.
(Clients will be able to use the volumes during the upgrade.)
Note: You should consider permanently adding a second controller to your SAN. Losing
a metadata controller without a standby can result in the loss of all data on a volume
and means that your volumes are not available during controller upgrades. Including a
standby controller in your SAN configuration is highly recommended.