Accusys ExaSAN SW-08 - Manual (Page 54)

6. Xsan
User Guide Page 6-3
6.4 Xsan Storage Breakdown
Below are more detailed explanations of each Xsan storage element and how you combine them to create
a shared Xsan volume.
6.4.1 LUNs
The smallest storage element you work with in Xsan is a logical storage device called a SCSI (Small
Computer System Interface) LUN (logical unit number). A LUN represents a group of drives combined
into a RAID array.
You create a LUN when you create a RAID array on a RAID storage device. The RAID system combines
physical drives into an array based on the RAID scheme you choose. Each array appears on the host as a
LUN.
If the RAID levels on your ExaSAN PCIe RAID systems (ExaSAN storage) aren’t right, you can recreate
LUNs based on other RAID levels or different numbers of drive modules. For more information about
RAID schemes, see Chapter 5, “RAID Overview.”
The illustration in Section 6.3 shows three LUNs. The LUN that stores metadata and journal information
uses RAID level 1(mirrored) to prevent metadata loss. The other data LUNs use RAID 5 (distributed
parity) for high performance and storage efficiency with data protection. Xsan sees the RAID arrays as
LUNs that can be combined to create an Xsan volume.
Your RAID LUNs are labeled and initialized for use with the Xsan file system when you use the Xsan
Admin to set up a volume.
6.4.2 Storage Pools
LUNs are combined to form storage pools. A storage pool might consist of a single LUN. A larger
volume might consist of several storage pools where each of which includes several LUNs.
Xsan distributes file data in parallel across the LUNs in a storage pool using a striping scheme. So you
can improve a client’s access speed by distributing available storage over several LUNs in a storage pool.
The illustration in Section 6.3 shows two Data LUNs combined into a storage pool for user data. This
storage pool uses 2 RAID 5 arrays (7 data disks + 1 parity disk), and Xsan stripes data across the LUNs in
the storage pool.