The Delphix Engine links to source physical databases via standard APIs and asks the source databases to send copies of their entire file and log blocks to it. The Delphix Engine uses intelligent filtering and compression to reduce the copy of the source database down to as little as 25% of the original size. The copy of the source database stored in the Delphix Engine, along with all incremental updates, is referred to as the dSource in Delphix terminology.After the initial loading, the Delphix Engine maintains synchronization with source databases based on policy - for example, once daily, or within seconds of the last transaction. Once linked, Delphix maintains a Timeflow of the source database - a rolling record of file and log changes retained by a policy (for example, "keep for two weeks.") From any time within that retention window, a virtual database (referred to in Delphix terminology as a VDB) can be instantly provisioned from the Delphix Engine. VDBs are served from the shared storage footprint of the dSource database Timeflow, so no additional storage is required.