Many distributed systems, however, choose to use a layered approach which abstracts the physical storage and then implement the object mapping in a higher layer. An example is the Frangipani [7] / Petal [5] distributed file system. The lower layer, called Petal, is a distributed virtual disk. Petal is a disk as opposed to a filesystem in that it only provides access to a huge linear array of bytes. There is no such concept as a file or object within Petal.
Frangipani, the file system, is implemented on top of Petal. It sees Petal as a huge virtual disk (containing 2 64 bits of data) within which to store its data.
PerDiS [3] is a Persistent Distributed Store which also uses the layered approach. In PerDiS the lower layer is a Distributed File System, while the upper layer contains ``Object support''.
Section 2.1.1 describes our Virtual Disk.