Apple Xsan 1.0 - Manual (Page 54)

54 Chapter 3 Managing SAN Storage
Setting the Block Allocation Size
The default file system block allocation size (4KB) is adequate for most volumes.
However, you can adjust this value along with the stripe breadth of the volume’s
storage pools to tune performance for special applications. For help choosing a block
allocation size, see “Choosing Block Allocation Size for a Volume,” below.
To set the block allocation size:
1 In Xsan Admin, select the SAN the volume belongs to, click Setup, and click Storage.
2 Double-click the volume in the list.
3 Use the up and down arrows to choose a new value for Block Allocation Size.
4 Click OK, then click Save.
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 16KB blocks of data, you can try
adjusting the block allocation size to 16KB. Then calculate the best corresponding stripe
breadth for the volume’s storage pools using this formula:
stripe breadth (number of blocks) = transfer size (bytes) / block allocation size (bytes)
For Xserve RAID systems, which have an optimal transfer size of 1 MB, this becomes:
stripe breadth = 1048576 / block allocation size
For the block allocation size of 16KB in our example, solving the equation
(1048576/16384) gives you a stripe breadth of 64.
Warning: When you change a volume’s block allocation size, all data on the volume is
lost.
LL0192.book Page 54 Thursday, July 29, 2004 5:20 PM