One of the questions I was often asked about resource pools (RP) is ‘Expandable reservation’. What is expandable reservation, and why should I care about it? Although it sounds intuitive, it can be easily misunderstood.
To put it simply, a resource pool with ‘expandable reservation’ can expand its reservation by asking more resources from its parent .
The need to expand reservation comes from the increase in reservation demand of its child objects (VMs or resource pools). If the parent resource pool is short of resources, then the parent expands it reservation asking resources from the grand parent.
Let us try to understand this with a simple example. Consider the following RP hierarchy. If RP-4 has to expand its reservation, it requests resources from its parent RP-3 and if RP-3 has to expand resources it eventually requests Root-RP.