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 on how to calculate the NovaReservedHostMemory value, see 「Calculating reserved host memory on Compute nodes」.
For information on how to determine the RHEL_min_swap value, see Recommended system swap space in the RHEL Managing Storage Devices guide.
