Apple Xsan 1.4 - Manual (Page 98)

98 Chapter 3 Managing SAN Storage
Choosing Block Allocation Size for a Volume
Xsan uses the volume block allocation size and storage pool stripe breadth together to
decide how to write data to a volume. For most SANs, the default values for volume
block allocation size and storage pool stripe breadth 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.
If the critical application that uses the volume reads and writes small blocks of data,
you might improve performance by choosing a correspondingly small allocation block
size. If, for example, the application reads and writes 16 KB blocks of data, you can try
adjusting the block allocation size to 16 KB. Then calculate the best corresponding
stripe breadth for the volume’s storage pools using this formula:
stripe breadth (blocks) = transfer size (bytes) / block allocation size (bytes)
For Xserve RAID systems, which have an optimal transfer size of 1 MB when used in
conjunction with Mac OS X or Mac OS X Server, this becomes:
stripe breadth = 1048576 / block allocation size
For the block allocation size of 16 KB in the example, solving the equation (1048576/
16384) gives a stripe breadth of 64.