One embodiment of the present invention is directed to providing a software layer that provides a Content Addressable Storage (CAS) capability in a computer system in which the content units are ultimately stored on a block I/O storage system. An application program may issue access requests to content units referring to them via a content address, and the software layer can convert such access requests to block I/O commands to be processed by the block I/O storage system. Thus, a CAS capability can be provided despite the absence of a storage system that provides such a capability natively.