User guide

Chapter 1 Overview of Xsan 19
In the illustration on page 16, the Other folder has an anity for the faster storage
pool that is based on a RAID 0 array. Any le 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 les and le 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 les in an Xsan volume using metadata les and
le system journals. File system metadata includes information such as which specic
parts of which disks are used to store a le and whether the le is being accessed. The
journal data includes a record of le system transactions that help ensure the integrity
of les in the event of a failure.
These les 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 rst
storage pool you add to a volume.
Striping at a Higher Level
When a RAID system writes a le using a RAID 0 (striping) scheme, it breaks the le
into segments and spreads them across disk drives in the RAID array. This improves
performance by writing parts of the le 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 le 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 Â
computer from modifying data on the volume.
Specify owner, group, and general access permissions in Xsan Admin. Â
Specify owner, group, and general access permissions in the Finder. Â