US Patent 8412688 Delegated reference count base file versioning

A snapshot copy facility maintains information indicating ownership and sharing of child nodes in the hierarchy of a file between successive versions by delegating reference counts to the parent-child relationships between the nodes, as indicated by pointers in the parent nodes. When a child node becomes shared between a parent node of the production file and a parent node of a snapshot copy, the delegated reference count is split among the parent nodes. This method is compatible with a conventional data de-duplication facility, and avoids a need to update reference counts in metadata of child nodes of a shared intermediate node upon splitting the shared intermediate node when writing to a production file.