One way to maximize the flexibility of storage allocation is to manage the disks in a disk group in units of a single capacity or of a small number of discrete capacities. This maximizes SFW's flexibility to allocate storage when new subdisks are required for new volumes, for volume extension, or for moving a subdisk from one disk to another.

To ensure that the amount of unallocated storage in each disk group is adequate, an appropriate level of unallocated storage be maintained. The distribution of unallocated storage across disks must allow for management operations such as failure-tolerant volume expansion to be carried out without violating volume failure tolerance and performance restrictions.

For example, if an additional mirror must be added to a mirrored striped volume, each subdisk of the added mirror must be located either on the same disk as the subdisk it extends, or on a disk separate from any of the volume's existing subdisks. (A subdisk is defined as a number of consecutively addressed blocks on a disk.) Subdisks are created by SFW as building blocks from which volumes are created. When an administrator makes a request to extend a volume, SFW checks the unallocated space in the disk group containing the volume to make sure that extension is possible. An administrator must maintain a distribution of unallocated capacity that allows such operations.