Chapter 2. Configuring the Compute (nova) service
Use environment files to customize the Compute (nova) service. Puppet generates and stores this configuration in the /var/lib/config-data/puppet-generated/<nova_container>/etc/nova/nova.conf file. Use the following configuration methods to customize the Compute service configuration:
Heat parameters - as detailed in the Compute (nova) Parameters section in the Overcloud Parameters guide. For example:
parameter_defaults: NovaSchedulerDefaultFilters: AggregateInstanceExtraSpecsFilter,RetryFilter,ComputeFilter,ComputeCapabilitiesFilter,ImagePropertiesFilter NovaNfsEnabled: true NovaNfsShare: '192.0.2.254:/export/nova' NovaNfsOptions: 'context=system_u:object_r:nfs_t:s0' NovaNfsVersion: '4.2'
Puppet parameters - as defined in
/etc/puppet/modules/nova/manifests/*:parameter_defaults: ComputeExtraConfig: nova::compute::force_raw_images: TrueNoteOnly use this method if an equivalent heat parameter does not exist.
Manual hieradata overrides - for customizing parameters when no heat or Puppet parameter exists. For example, the following sets the
disk_allocation_ratioin the[DEFAULT]section on the Compute role:parameter_defaults: ComputeExtraConfig: nova::config::nova_config: DEFAULT/disk_allocation_ratio: value: '2.0'
If a heat parameter exists, it must be used instead of the Puppet parameter; if a Puppet parameter exists, but not a heat parameter, then the Puppet parameter must be used instead of the manual override method. The manual override method must only be used if there is no equivalent heat or Puppet parameter.
Follow the guidance in Identifying Parameters to Modify to determine if a heat or Puppet parameter is available for customizing a particular configuration.
See Parameters in the Advanced Overcloud Customization guide for further details on configuring overcloud services.
2.1. Configuring memory for overallocation
When you use memory overcommit (NovaRAMAllocationRatio >= 1.0), you need to deploy your overcloud with enough swap space to support the allocation ratio.
If your NovaRAMAllocationRatio parameter is set to < 1, follow the RHEL recommendations for swap size. For more information, see Recommended system swap space in the RHEL Managing Storage Devices guide.
Prerequisites
- You have calculated the swap size your node requires. For more information, see Section 2.3, “Calculating swap size”.
Procedure
Copy the
/usr/share/openstack-tripleo-heat-templates/environments/enable-swap.yamlfile to your environment file directory:$ cp /usr/share/openstack-tripleo-heat-templates/environments/enable-swap.yaml /home/stack/templates/enable-swap.yaml
Configure the swap size by adding the following parameters to your
enable-swap.yamlfile:parameter_defaults: swap_size_megabytes: <swap size in MB> swap_path: <full path to location of swap, default: /swap>
To apply this configuration, add the
enable_swap.yamlenvironment file to the stack with your other environment files and deploy the overcloud:(undercloud) $ openstack overcloud deploy --templates \ -e [your environment files] \ -e /home/stack/templates/enable-swap.yaml \
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_nowith the number of instances. -
Replace
avg_instance_sizewith the average amount of memory each instance can use. -
Replace
overheadwith the hypervisor overhead required for each instance. -
Replace
resource1with the number of a resource type on the node. -
Replace
resource_ramwith the amount of RAM each resource of this type requires.
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 Section 2.2, “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.
