Apple Xsan 2 (Third Edition) - Manual (Page 17)

Chapter 1 Overview of Xsan 17
The illustration on page 16 shows eight LUNs. The LUN that stores metadata and
journal information uses RAID level 1 (mirrored) to prevent metadata loss. One LUN
stores users’ data on a RAID 0 array (striping only) for best speed and storage efficiency
but no data protection. 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 a volume.
Your RAID LUNs are labeled and initialized for use with the Xsan file system when you
use Xsan Admin to set up a volume.
Storage Pools
LUNs are combined to form storage pools. A storage pool in a small volume might
consist of a single RAID array, but a larger volume might consist of several storage
pools each of which includes several arrays.
Xsan distributes file data in parallel across the LUNs in a storage pool using a RAID 0
(striping) scheme. So, you can improve a client’s access speed by distributing available
storage over several LUNs in a storage pool.
You can set up storage pools that have different performance or recoverability
characteristics based on the RAID level of their LUNs, and assign folders to them using
affinities. Users can then select where to store files based on their need for speed or
safety. See “Folders with Affinities” on page 18.
The illustration on page 16 shows eight LUNs combined into five storage pools for user
data. One pool uses a single RAID 0 array (fast, but not recoverable). Three other pools
use multiple RAID 5 arrays (not as fast, but recoverable), and Xsan stripes data across
the LUNs in each storage pool.
You use Xsan Admin to add available LUNs to storage pools.
Affinities and Affinity Tags
An affinity associates a folder with particular storage pools. When you assign an affinity
to a folder, you guarantee that files placed in the folder are stored on a storage pool
that has the corresponding affinity tag.
An affinity tag groups storage pools together based on performance and
recoverability characteristics. More than one storage pool can have the same affinity
tag. Xsan distributes the contents of a folder with a specific affinity among the storage
pools that have that affinity tag. This strategy improves performance when users
simultaneously read and write files in the same folder, because the read and write
operations are distributed among the storage pools and their component LUNs.
You use Xsan Admin to assign affinity tags to storage pools and associate folders with
those affinity tags.