Apple Xsan 1.1 - Manual (Page 26)

26 Chapter 2 Setting Up a Storage Area Network
Review the considerations and guidelines on the following pages for help translating
your answers into a suitable SAN design.
Planning Considerations and Guidelines
The following paragraphs might help you make some of your SAN design decisions.
How Much Storage?
Because it’s easy to add storage to an Xsan SAN, you only need to decide on an
adequate starting point. You can then add storage for user data as needed.
You can’t expand a storage pool that is used to store volume metadata and journal
data. For information on estimating your metadata and journal data storage
requirements, see “Estimating Metadata and Journal Data Storage Needs” on page 31.
Note that the number of Xserve RAID systems you use affects not only available space
but also SAN performance. See “Performance Considerations” below.
How Should Users See Available Storage?
If you want the users working on a particular project to see a volume dedicated to their
work, create a separate volume for each project. If it’s acceptable for a user to see a
folder for his or her work on a volume with others’ folders, you can create a single
volume and organize it into project folders.
Workflow Considerations
How much file sharing is required by your users’ workflow? If, for example, different
users or groups work on the same files, either simultaneously or in sequence, it makes
sense to store those files on a single volume to avoid having to maintain or hand off
copies. Xsan uses file locking to manage shared access to a single copy of the files.
Performance Considerations
If your SAN supports an application (such as high resolution video capture and
playback) that requires the fastest possible sustained data transfers, design your SAN
with these performance considerations in mind:
 Set up the LUNs (RAID arrays) using a RAID scheme that offers high performance.
See “Choosing RAID Schemes for LUNs” on page 28.
 Group your fastest LUNs in storage pools reserved for the application. Reserve slower
devices for a volume dedicated to less demanding or supporting applications.
 To increase parallelism, spread LUNs across different Xserve RAID controllers. For
example, instead of creating a single 4-disk LUN on one side of an Xserve RAID,
create two 2-disk LUNs, one on each side, and add these LUNs to a storage pool.
Xsan then stripes data across the two LUNs and benefits from simultaneous transfers
through two controllers.
 To increase parallelism in a relatively small storage pool (the size of one or a few
drive modules), create a slice of similar size across all the drives on a controller
instead of creating the storage pool from just one or two drive modules.