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The storage numbers are always calculated the same way for Virtual Datacenters:
Storage Used = Total allocated disk space of VMs deployed in Org VDC + Total memory of VMs deployed* in Org VDC (to account for VM swap usage) + Maximum possible existing snapshot size
Storage allocated = Org VDC storage allocation
Example 1: If you have 1 VM with 1 GB RAM and 10 GB disk the storage usage will be: 10 GB (disk) + 1 GB (VM swap) = 11 GB.
Example 2: If you have 1 VM with 1 GB RAM and 10 GB disk and create a snapshot (including memory), the storage usage will be: 10 GB (disk) + 1 GB (VM swap) + 10 GB (maximum disk snapshot size) + 1 GB (memory snapshot size) = 22 GB.
* Virtualisation Platforms stores VM Memory as files on storage to increase resiliency in HA scenarios and during periods of high usage.