Apple Xsan 2 (Third Edition) - Manual (Page 19)

Chapter 1 Overview of Xsan 19
In the illustration on page 16, the Other folder has an affinity for the faster storage
pool that is 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 and journal data are stored on the first
storage pool you add to a volume.
Striping 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 individual 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”).
Security
There are several ways you can control access to a SAN volume:
Unmount a volume on client computers that shouldn’t have access to it. Users can’t Â
browse or mount SAN volumes; only a SAN administrator can mount SAN volumes
on clients.
Mount a volume on a client for read-only access, to prevent users on a client
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computer from modifying data on the volume.
Specify owner, group, and general access permissions in Xsan Admin.
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Specify owner, group, and general access permissions in the Finder. В