• Fixed provisioning The size of the virtual disk is the actual amount of physical storage space allocated from the pool.
• Thin provisioning The size of the virtual disk represents the maximum amount of physical storage space that can be allocated from the pool. No space is actually used, however, until data is stored on a volume on the virtual disk, and the amount of space used will grow or shrink as data is written to or deleted from the disk.

подробнее:

http://tutorial.wmlcloud.com/windows_server/windows-server-2012—deploying-storage-spaces-(part-2)—understanding-storage-spaces—fixed-vs_-thin-provisioning.aspx

Ну и немного дискуссий:

http://habrahabr.ru/company/netapp/blog/108534/