VMware vSphere vCenter Server 4.0 - Manual (Page 42)

Table 4-2. Reservation Types
Reservation Type Description
Fixed The system checks whether the selected resource pool has sufficient unreserved
resources. If it does, the action can be performed. If it does not, a message appears and
the action cannot be performed.
Expandable
(default)
The system considers the resources available in the selected resource pool and its direct
parent resource pool. If the parent resource pool also has the Expandable Reservation
option selected, it can borrow resources from its parent resource pool. Borrowing
resources occurs recursively from the ancestors of the current resource pool as long as
the Expandable Reservation option is selected. Leaving this option selected offers more
flexibility, but, at the same time provides less protection. A child resource pool owner
might reserve more resources than you anticipate.
The system does not allow you to violate preconfigured Reservation or Limit settings. Each time you
reconfigure a resource pool or power on a virtual machine, the system validates all parameters so all service-
level guarantees can still be met.
Expandable Reservations Example 1
This example shows you how a resource pool with expandable reservations works.
Assume an administrator manages pool P, and defines two child resource pools, S1 and S2, for two different
users (or groups).
The administrator knows that users want to power on virtual machines with reservations, but does not know
how much each user will need to 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. S1 can start using all of P's reservation, so that no
memory or CPU is directly available to S2.
Expandable Reservations Example 2
This example shows how a resource pool with expandable reservations works.
Assume the following scenario (shown in
Figure 4-3 ).
n Parent pool RP-MOM has a reservation of 6GHz and one running virtual machine VM-M1 that reserves
1GHz.
n You create a child resource pool RP-KID with a reservation of 2GHz and with Expandable Reservation
selected.
n You add two virtual machines, VM-K1 and VM-K2, with reservations of 2GHz each to the child resource
pool and try to power them on.
n VM-K1 can reserve the resources directly from RP-KID (which has 2GHz).
n No local resources are available for VM-K2, so it borrows resources from the parent resource pool, RP-
MOM. RP-MOM has 6GHz minus 1GHz (reserved by the virtual machine) minus 2GHz (reserved by RP-
KID), which leaves 3GHz unreserved. With 3GHz available, you can power on the 2GHz virtual machine.
vSphere Resource Management Guide
42 VMware, Inc.