Apple Xsan 2 (Third Edition) - Manual (Page 31)

Chapter 2 Planning a Storage Area Network 31
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.
Choosing 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.
Storing 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. They’re stored on the first storage pool in the
volume.
Preset volume types set up a separate storage pool used only for metadata and journal
data. If you use Xserve RAID systems for storage, make sure that the LUNs you assign
to the metadata and journal storage pool are connected to a different RAID controller
than the LUNs you assign to affinity tags for user 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.
Estimating Metadata and Journal Data Storage Needs
To estimate the amount of space required for Xsan volume metadata, assume that
10 million files on a volume require approximately 10 GB of metadata on the volume’s
metadata storage pool.
Choosing an Allocation Strategy
If you choose a preset volume type when you set up a volume, Xsan Admin sets its
volume allocation strategy for you. Later, you can change the allocation strategy by
editing volume settings with Xsan Admin. The allocation strategy you choose for a
volume determines the order in which its storage pools are filled with data. You can
choose round robin, fill, or balance:
 If you choose round robin, Xsan writes data to each storage pool in the volume in
turn. This is normally the best choice for performance.
 If you choose fill, Xsan writes data to the first storage pool in the volume until that
storage pool is full, and then moves to the next storage pool. This is a good choice
to keep a specific storage pool unused as long as possible.