OpenShift Virtualization is an add-on to OpenShift Container Platform and imposes additional overhead that you must account for when planning a cluster. Each cluster machine must accommodate the following overhead requirements in addition to the OpenShift Container Platform requirements. Oversubscribing the physical resources in a cluster can affect performance.
The numbers noted in this documentation are based on Red Hat’s test methodology and setup. These numbers can vary based on your own individual setup and environments.
Calculate the memory overhead values for OpenShift Virtualization by using the equations below.
Cluster memory overhead
Memory overhead per infrastructure node ≈ 150 MiB
Memory overhead per worker node ≈ 360 MiB
Additionally, OpenShift Virtualization environment resources require a total of 2179 MiB of RAM that is spread across all infrastructure nodes.
Virtual machine memory overhead
Memory overhead per virtual machine ≈ (1.002 * requested memory) + 146 MiB \
+ 8 MiB * (number of vCPUs) \ (1)
+ 16 MiB * (number of graphics devices) (2)
||Number of virtual CPUs requested by the virtual machine
||Number of virtual graphics cards requested by the virtual machine
If your environment includes a Single Root I/O Virtualization (SR-IOV) network device or a Graphics Processing Unit (GPU), allocate 1 GiB additional memory overhead for each device.
Calculate the cluster processor overhead requirements for OpenShift Virtualization by using the equation below. The CPU overhead per virtual machine depends on your individual setup.
Cluster CPU overhead
CPU overhead for infrastructure nodes ≈ 4 cores
OpenShift Virtualization increases the overall utilization of cluster level services such as logging, routing, and monitoring. To account for this workload, ensure that nodes that host infrastructure components have capacity allocated for 4 additional cores (4000 millicores) distributed across those nodes.
CPU overhead for worker nodes ≈ 2 cores + CPU overhead per virtual machine
Each worker node that hosts virtual machines must have capacity for 2 additional cores (2000 millicores) for OpenShift Virtualization management workloads in addition to the CPUs required for virtual machine workloads.
Virtual machine CPU overhead
If dedicated CPUs are requested, there is a 1:1 impact on the cluster CPU overhead requirement. Otherwise, there are no specific rules about how many CPUs a virtual machine requires.
Use the guidelines below to estimate storage overhead requirements for your OpenShift Virtualization environment.
Cluster storage overhead
Aggregated storage overhead per node ≈ 10 GiB
10 GiB is the estimated on-disk storage impact for each node in the cluster when you install OpenShift Virtualization.
Virtual machine storage overhead
Storage overhead per virtual machine depends on specific requests for resource allocation within the virtual machine. The request could be for ephemeral storage on the node or storage resources hosted elsewhere in the cluster. OpenShift Virtualization does not currently allocate any additional ephemeral storage for the running container itself.
As a cluster administrator, if you plan to host 10 virtual machines in the cluster, each with 1 GiB of RAM and 2 vCPUs, the memory impact across the cluster is 11.68 GiB. The estimated on-disk storage impact for each node in the cluster is 10 GiB and the CPU impact for worker nodes that host virtual machine workloads is a minimum of 2 cores.