Assume an administrator manages a parent pool P1, and defines two child resource pools, C1 and C2, for two different groups of users. C1 and C2 are self-serve resource pools, allowing users to configure their own virtual machines. The administrator does not know in advance exactly what resources the users will need, so the administrator wants the resource pool configuration to be flexible.
Without expandable reservations, the administrator needs to explicitly allocate fixed amounts of resources to C1 and C2. Such specific allocations can be inflexible, especially in deep resource pool hierarchies, and can complicate setting reservations for C1 and C2. By making the reservations for C1 and C2 expandable, the administrator allows users to more flexibly share and inherit the common reservation for pool P1.
Expandable reservations cause a loss of resource pool isolation in the context of admission control. For example, if C1 and C2 have their reservation amounts set to 0, then virtual machines in C1 might use all of P1's memory reservation, so that no memory is available to start virtual machines in C2.
