Disco  introduced transparent page sharing as a method for eliminating redundant copies of pages, such as code or read-only data, across virtual machines. Once copies are identified, multiple guest ``physical'' pages are mapped to the same machine page, and marked copy-on-write. Writing to a shared page causes a fault that generates a private copy.
Unfortunately, Disco required several guest OS modifications to identify redundant copies as they were created. For example, the bcopy() routine was hooked to enable file buffer cache sharing across virtual machines. Some sharing also required the use of non-standard or restricted interfaces. A special network interface with support for large packets facilitated sharing data communicated between VMs on a virtual subnet. Interposition on disk accesses allowed data from shared, non-persistent disks to be shared across multiple guests.