The administrator knows that users want to power on virtual machines with reservations, but does not know how much each user will reserve. Making the reservations for S1 and S2 expandable allows the administrator to more flexibly share and inherit the common reservation for pool P.

Without expandable reservations, the administrator needs to explicitly allocate S1 and S2 a specific amount. Such specific allocations can be inflexible, especially in deep resource pool hierarchies and can complicate setting reservations in the resource pool hierarchy.

Expandable reservations cause a loss of strict isolation; that is, S1 can start using all of P's reservation, so that no memory or CPU is directly available to S2.