Apple Xsan 1.4 - Manual (Page 24)

24 Chapter 1 Overview of Xsan
The following paragraphs describe these storage elements and how you organize them
to create shared Xsan volumes.
LUNs
The smallest storage element you work with in Xsan is a logical storage device called a
LUN (a SCSI logical unit number). In most storage area networks a LUN represents a
group of drives such as a RAID array or a JBOD (just a bunch of disks) device. In Xsan,
LUNs are Xserve RAID arrays or slices.
You create a LUN when you use the RAID Admin application to create an Xserve RAID
array. The controller hardware and software in the Xserve RAID system combine
individual drive modules into an array based on the RAID scheme you choose. Each
array appears on the network as a separate LUN. If you slice an array, each slice appears
as a LUN.
One of your first tasks when you set up a SAN volume is to prepare LUNs. If the two
RAID 5 arrays on a new Xserve RAID are not right for your application, you can use
RAID Admin to create arrays based on other RAID schemes. For help choosing schemes
for your LUNs, see “Choosing RAID Schemes for LUNs” on page 44.
The illustration on page 23 shows four Xserve RAID systems that each host two arrays.
Half of the arrays use a RAID 0 scheme (striping only) for speed while the others use
RAID 5 (distributed parity) to ensure against data loss. Xsan sees the arrays as LUNs that
can be combined to create a volume.
After your Xserve RAID LUNs are set up, you label and initialize them for use with the
Xsan file system using Xsan Admin.