Block Allocation Size:
The allocation strategy you choose for a volume determines the order in which its storage pools are filled with data. You can choose round robin, fill, or balance.
If you choose round robin, Xsan writes new data in turn to each storage pool in the volume.
If you choose fill, Xsan writes all new data to the first storage pool in the volume until that storage pool is full, and then moves to the next storage pool. This is a good choice if you want to keep a particular storage pool unused as long as possible.
If you choose balance, Xsan writes new data to the storage pool with the most free space.