Apple Xsan 2 - Manual (Page 48)

LUNs configured as RAID 0 arrays (striping only) or LUNs based on single drives are
difficult or impossible to recover if they fail. Unprotected LUNs such as these should
be used only in storage pools that store scratch files or other data that you can afford
to lose.
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
Decide on the number of volumes
A volume is the largest unit of shared storage on the SAN. If 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, remember you can’t control client access by unmounting
volumes on Xsan 2.3 clients. Users whose computers have Mac OS X Lion or Lion
Server can mount SAN volumes themselves.
For a typical balance of security and shared access, create one volume and control
access with folder access privileges or ACLs in Xsan Admin or the Server app.
Decide 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.
Assign 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.
48 Chapter 3 Plan a SAN