Apple Xsan 1.1 - Manual (Page 7)

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General Information About
Upgrading
This chapter contains general information that you
should be aware of before you upgrade your Xsan
storage area network (SAN).
Mac OS X or Mac OS X Server?
The Xsan file system, the Xsan Admin application, and the Xsan User Quotas
application run identically on Mac OS X and Mac OS X Server. Any statement in this
guide about Mac OS X also applies to Mac OS X Server.
Migrating Without Interruption
The instructions in this guide show you how to upgrade without interfering with the
availability of your Xsan volumes. If you follow the instructions, clients and applications
can access the SAN volumes during the upgrade.
This constant availability is based on the ability of the Xsan software to switch control
of a volume from one metadata controller to another if the first becomes unresponsive.
This process, called “failover,” requires you to have at least two metadata controllers in
your SAN.
Note: Even if you have two controllers in your SAN, there will be a brief period (when
the controller you are upgrading is restarting) during which your SAN has only one
functioning controller. If you can’t afford to risk this single point of failure even for a
short time, you should temporarily add a third controller to your SAN. You can do this
by temporarily promoting a suitable client computer to the role of controller. See
“Promoting a Client to Standby Controller” on page 9.