Apple Xsan 2 (Third Edition) - Manual (Page 29)

Chapter 2 Planning a Storage Area Network 29
Most RAID systems support all popular RAID levels. Each RAID scheme offers a different
balance of performance, data protection, and storage efficiency, as summarized in the
following table.
RAID level Storage efficiency Read
performance
Write
performance Data protection
RAID 0 Highest Very High Highest No
RAID 1 Low High Medium Yes
RAID 3 High to very high Medium Medium Yes
RAID 5 High to very high High High Yes
RAID 0+1 Low High High Yes
Deciding on the Number of Volumes
A volume is the largest unit of shared storage on the SAN. If your users need shared
access to files, store those files on the same volume. This makes it unnecessary for
them to pass copies of the files among themselves.
However, if security is critical, one way to control client access is to create separate
volumes and unmount volumes on clients that shouldn’t have access to them.
For a more typical balance of security and shared access, a flexible compromise is to
create one volume and control access with folder access privileges or ACLs in Xsan
Admin (or in Mac OS X Server’s Server Admin).
Deciding How to Organize a Volume
You can help users organize data on a volume or restrict users to specific areas of
the volume by creating predefined folders. You can control access to these folders by
assigning access permissions using Xsan Admin.
You can assign folders to specific storage pools using affinities. For example, you can
create a folder for data that requires fast access and assign that folder to your fastest
storage pool.
Assigning LUNs to Affinity Tags
When you create a volume using a preset volume type that fits your SAN scenario,
Xsan Admin sets up storage pools and affinity tags for best performance. All you do
is assign LUNs to each affinity tag. Xsan Admin determines the optimal number of
storage pools to create, based on the volume type and the number of LUNs you assign
to each affinity tag.
For best performance, assign LUNs in the multiples shown below. These multiples
apply to affinity tags used for user data, not to the Metadata and Journal affinity tag,
which needs just one LUN.