Apple Xsan 1.0 - Manual (Page 13)

Chapter 1 Introduction to Xsan 13
How Xsan Utilizes Available Storage
Xsan stores both 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 particular file and whether the file is being
accessed. The journal data includes a record of file system transactions that can 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. By default, metadata and journal data are stored
on the first storage pool you add to a volume. You can use Xsan Admin to choose
where these files are stored when you add storage pools to a new volume.
Striping at a Higher Level
When you write a file to a RAID array using RAID 0 (striping), the file is broken into
segments that are spread across the individual disk drives in the array. This improves
performance by writing pieces of the file in parallel (instead of one piece at a time) to
the individual disks in the array. Xsan applies this same technique at a second, higher
level 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. Once again,
performance is improved because data is written in parallel.
You can tune SAN performance by adjusting the amount of data written to each LUN in
a storage pool (the “stripe breadth”) to suit a critical application.
Security
As SAN administrator, you can control access to shared volumes in several ways.
First, users cannot browse or mount SAN volumes. Only a SAN administrator can
specify which volumes are mounted on which client computers. One way you can
control access to data is to mount a volume only on appropriate client computers.
To prevent users from modifying data on a volume, you can mount the volume with
read-only access.
You can also control user access to folders on a volume by specifying owner, group,
and general access permissions as you would in the Finder.
You can also set up zones in the underlying Fibre Channel network to segregate users
and volumes.
LL2652.book Page 13 Wednesday, July 28, 2004 3:45 PM