All volumes have a block size. In Mac OS X you can see a block size by entering the following command (assuming disk0): sudo diskutil information 0 If you notice in the Total Size: field you’ll see the number of blocks followed by a block size. By default it is 512 bytes. One of the things that makes it possible to attain high performance from an Xsan volume is the ability for the block size to be customized. The general rule of thumb is that the Stripe Breadth and the Block size should multiply to equal 1MB. So if you have a block size of 256 then your stripe breadth should be 4.