Can I Create a Single Server Using the Over-Allocated Resources?

Problem

A physical host has 1000 GB RAM and the allocation ratio has been set to 1.5. Can we create a single instance that uses these over-committed resources entirely (1500 GB)?

Environment

• Platform9 Managed OpenStack - All versions
• Nova

Answer

For overcommitment, a host needs to be able to run a single instance at full capacity if needed which essentially means if the host has 1 GB RAM with an allocation ratio of 10, you can run 10 VMs with 1GB each, but not a single VM with 10GB. This is because that single VM can never really use the overcommitted 10GB on the physical host but the 10 VMs could always time share a host at their max capacity, even though not simultaneously.