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2.2. Calculating reserved host memory on Compute nodes

To determine the total amount of RAM to reserve for host processes, you need to allocate enough memory for each of the following:

  • The resources that run on the node, for instance, OSD consumes 3 GB of memory.
  • The emulator overhead required to visualize instances on a host.
  • The hypervisor for each instance.

After you calculate the additional demands on memory, use the following formula to help you determine the amount of memory to reserve for host processes on each node:

NovaReservedHostMemory = total_RAM - ( (vm_no * (avg_instance_size + overhead)) + (resource1 * resource_ram) + (resource _n_ * resource_ram))
  • Replace vm_no with the number of instances.
  • Replace avg_instance_size with the average amount of memory each instance can use.
  • Replace overhead with the hypervisor overhead required for each instance.
  • Replace resource1 with the number of a resource type on the node.
  • Replace resource_ram with the amount of RAM each resource of this type requires.