2.3. Calculating swap size

The allocated swap size must be large enough to handle any memory overcommit. You can use the following formulas to calculate the swap size your node requires:

  • overcommit_ratio = NovaRAMAllocationRatio - 1
  • Minimum swap size (MB) = (total_RAM * overcommit_ratio) + RHEL_min_swap
  • Recommended (maximum) swap size (MB) = total_RAM * (overcommit_ratio + percentage_of_RAM_to_use_for_swap)

The percentage_of_RAM_to_use_for_swap variable creates a buffer to account for QEMU overhead and any other resources consumed by the operating system or host services.

For instance, to use 25% of the available RAM for swap, with 64GB total RAM, and NovaRAMAllocationRatio set to 1:

  • Recommended (maximum) swap size = 64000 MB * (0 + 0.25) = 16000 MB

For information about how to calculate the NovaReservedHostMemory value, see Calculating reserved host memory on Compute nodes.

For information about how to determine the RHEL_min_swap value, see Recommended system swap space in the RHEL Managing Storage Devices guide.