Apple Xsan 1.0 - Manual (Page 50)

50 Chapter 3 Managing SAN Storage
Setting Storage Pool Stripe Breadth
The default stripe breadth value (64 file system blocks) is adequate for storage pools in
most SAN volumes. However, you can adjust this value along with the file system block
allocation size to tune SAN performance for special applications. For help choosing a
stripe breadth, see “Choosing Stripe Breadth for a Storage Pool,” below.
To set the stripe breadth:
1 In Xsan Admin, select the SAN the storage pool belongs to, click Setup, and click
Storage.
2 Double-click the storage pool in the list.
3 Next to Stripe Breadth, type the new value.
4 Click OK, then click Save.
Choosing Stripe Breadth for a Storage Pool
Xsan uses the storage pool stripe breadth and volume block allocation size together to
decide how to write data to a volume. For most SANs, the default values for storage
pool stripe breadth and volume block allocation size result in good performance.
However, in some cases you might be able to improve read and write performance by
adjusting these values to suit a specific application.
The stripe breadth of a storage pool is the number of file allocation blocks that are
written to a LUN in the pool before moving to the next LUN. To choose an efficient
stripe breadth, you need to consider two other factors:
• The most efficient data transfer size of the LUN storage device (1 MB for Xserve RAID)
• The size of the data blocks written and read by the critical application that uses the
volume (as reflected in the block allocation size for the volume)
Knowing these values, choose a stripe breadth using this formula:
stripe breadth (in blocks) = transfer size (in bytes) / block allocation size (in bytes)
For Xserve RAID systems, which have an optimal transfer size of 1 MB, this becomes:
stripe breadth = 1048576 / block allocation size
If, for example, you are using an application such as Final Cut Pro to move large
amounts of video data, choose the largest block allocation size (512KB) and use the
equation to find the stripe breadth of 2 blocks. Then, Xsan writes 1 MB of data (two
512KB blocks), in turn, to each LUN in your video storage pool.
Warning: When you change a storage pool’s stripe breadth, all data on the storage
pool and the volume to which it belongs is lost.
LL0192.book Page 50 Thursday, July 29, 2004 5:20 PM