Virtualization methods and systems for optimizing the availability of items in an inventory of items in a reservation system. The items are classified into item types and an item type is defined by a requestable set of at least one characteristic. Reservations may be received for a set of at least one characteristic that is a subset of an item type. After a reservation has been accepted, all the availabilities of the requestable sets of at least one characteristic in the inventory are updated. The reservation system may be a hotel reservation system and item types may be hotel room types or other bookable products. The reservation system may be a flight reservation system and item types may be bookable places on flights.