Apple Xsan 1.1 - Manual (Page 29)

Chapter 2 Setting Up a Storage Area Network 29
Deciding How to Organize a Volume
You can help users organize data on a volume or restrict users to specific areas of the
volume by creating predefined folders. You can control access to these folders by
assigning access permissions using Xsan Admin.
You can assign folders to specific storage pools using affinities. You can, for example,
create a folder for data that requires fast access and assign that folder to your fastest
storage pool.
Assigning LUNs to Storage Pools
You should set up a storage pool using LUNs that have similar capacity and
performance characteristics.
To provide high performance, Xsan uses the RAID 0 scheme to stripe data across the
LUNs in a storage pool. This requires that the LUNs in the pool be the same size. If you
set up a storage pool using LUNs of different sizes, Xsan uses available space on each
LUN equal to the capacity of the smallest LUN. If the LUNs vary in size, this can result in
wasted capacity. For example, if you assign 240 GB and 360 GB RAID arrays to a storage
pool, 120 GB of the larger array will not be used. By combining LUNs with similar
capacities, you avoid wasting available storage.
If you want to set up a storage pool for use by a high performance application, assign
similarly high speed LUNs. Assign slower LUNs to a storage pool where you keep data
that doesn’t have critical performance requirements.
Creating storage pools from LUNs that are hosted on different drive modules and
different RAID controllers increases performance by increasing the parallelism of data
transfers. For example, a storage pool consisting of two LUNs, each a single drive
module on the left side of an Xserve RAID, will not be as fast as a similarly sized storage
pool made up of two LUNs that are single slices across all seven drives, one slice on
each controller. In the first case, all transfers go through a single RAID controller to just
two drives; in the second case the same transfer is spread across two RAID controllers
and fourteen drives.
Assigning Storage Pools to Volumes
After you decide how to combine available LUNs into storage pools, assign the storage
pools to the volumes you want to create.
For best performance, create separate storage pools for file system metadata and
journal data.
Note: No storage pool or volume can be larger than 16 TB (Mac OS X v10.3) or 1024 TB
(Mac OS X v10.4).