Apple Xsan 2 - Manual (Page 37)

Chapter 2 Overview of Xsan 37
For example, you can associate some folders with an affinity whose storage pools have
faster LUNs, and associate other folders with an affinity whose storage pools have
safer LUNs. Users can choose between faster and safer storage by putting files in the
appropriate folder.
In the illustration on page 34, the Other folder has an affinity for the faster storage
pool based on a RAID 0 array. Any file that a user copies into the Other folder is stored
on the faster array. The Video and Audio folders are associated with the more secure
RAID 5 storage.
How Xsan uses available storage
Xsan stores user files and file system data on SAN volumes, and stripes data across the
LUNs in a volume for better performance.
Metadata and journal data
Xsan records information about the files in an Xsan volume using metadata files and
file system journals. File system metadata includes information such as which specific
parts of which disks are used to store a file and whether the file is being accessed. The
journal data includes a record of file system transactions that help ensure the integrity
of files in the event of a failure.
These files are managed by the Xsan metadata controller but are stored on SAN
volumes, not on the controller itself. Metadata is stored on the first storage pool
you add to a volume. Journal data can also be stored on the same storage pool as
metadata, or you can use a separate storage pool for journal data. You must have
journal data on only one storage pool.
Stripe at a higher level
When a RAID system writes a file using a RAID 0 (striping) scheme, it breaks the file
into segments and spreads them across disk drives in the RAID array. This improves
performance by writing parts of the file in parallel (instead of one part at a time) to
disks in the array.
Xsan applies this same technique in the storage hierarchy. Within each storage pool
in a volume, Xsan stripes file data across the LUNs that make up the storage pool.
Performance is improved because data is written in parallel.
You can tune SAN performance to suit a critical application by adjusting the amount
of data written to each LUN in a storage pool (the stripe breadth).