Apple Xsan 2 - Manual (Page 50)

When you add a client to the SAN, volumes that are currently mounted on all
SAN computers will be mounted automatically on the new client. If a volume isn’t
mounted on all SAN computers, and you add a new client, the volume isn’t mounted
automatically on the new client. If a volume isn’t mounted on all SAN computers but is
mounted on all metadata controllers, and you add a metadata controller, the volume
is mounted automatically on the new metadata controller. If you stop a volume, it isn’t
mounted automatically on any SAN computers when you start the volume again.
You can use Xsan Admin to mount or unmount a volume on selected clients. Users
whose SAN computers have Mac OS X Lion or Lion Server can mount and unmount
SAN volumes themselves using the Xsan pane of System Preferences.
Choose metadata controllers
You must choose at least one computer to be the SAN metadata controller, the
computer that is responsible for managing file system metadata.
Note: File system metadata and journal data are stored on the SAN volume, not on the
metadata controller itself. For more information, see “Store user data with metadata
and journal data” below.
If high availability is important, use at least two metadata controllers: one as the
primary controller and one as a standby. You can specify additional metadata
controllers as needed, and set each volume’s failover priorities to determine the order
in which the controllers are tried if a volume’s primary controller stops responding.
If performance is critical, don’t run other server services on the metadata controller
and don’t use the controller to reshare a SAN volume using AFP or NFS.
Choose standby controllers
To be sure that SAN volumes are always available, set up at least one standby
metadata controller that can take over if your primary metadata controller fails.
Store user data with metadata and journal data
The metadata and journal data that describe a volume are stored not on the volume’s
metadata controller, but on the volume. Metadata is stored on the first storage pool in
the volume. Journal data can be stored on any storage pool in the volume. You must
have only one storage pool with journal data.
Preset volume types set up a separate storage pool used only for metadata and journal
data.
If you set up a custom volume with more than one storage pool, you can choose
whether the metadata and journal storage pool is allowed to store user data. You
might get adequate performance by combining metadata and journal data on the
same storage pool as user data, but for better performance, use a separate storage
pool for metadata and journal data.
50 Chapter 3 Plan a SAN