A vRealize Automation reservation can define policies, priorities, and quotas that determine machine placement for provisioning requests.
Reservation policies restrict machine provisioning to a subset of available reservations. Storage reservation policies allow blueprint architects to assign machine volumes to different datastores.
To provision successfully, the reservation must have sufficient available storage. The reservation's storage availability depends on:
How much storage is available on the datastore/cluster.
How much of that storage is reserved for that datastore/cluster.
How much of that storage is already allocated in vRealize Automation
For example, even if the vCenter Server has storage available for the datastore/cluster, if sufficient storage is not reserved in the reservation then provisioning fails with a "No reservation is available to allocate..." error. The allocated storage on a reservation depends on the number of VMs (regardless of their state) on that specific reservation. See the VMware Knowledge Base article Machine XXX: No reservation is available to allocate within the group XXX. Total XX GB of storage was requested (2151030) at http://kb.vmware.com/kb/2151030 for more information.