Chapter 4. Configuring Compute service storage

You create an instance from a base image, which the Compute service copies from the Image (glance) service, and caches locally on the Compute nodes. The instance disk, which is the back end for the instance, is also based on the base image.

You can configure the Compute service to store ephemeral instance disk data locally on the host Compute node or remotely on either an NFS share or Ceph cluster. Alternatively, you can also configure the Compute service to store instance disk data in persistent storage provided by the Block Storage (Cinder) service.

You can configure image caching for your environment, and configure the performance and security of the instance disks.

4.1. Configuration options for image caching

Use the parameters detailed in the following table to configure how the Compute service implements and manages an image cache on Compute nodes.

Table 4.1. Compute (nova) service image cache parameters

Configuration methodParameterDescription



Specifies the number of seconds to wait between runs of the image cache manager, which manages base image caching on Compute nodes. The Compute service uses this period to perform automatic removal of unused cached images when nova::compute::image_cache::remove_unused_base_images is set to True.

Set to 0 to run at the default metrics interval of 60 seconds (not recommended). Set to -1 to disable the image cache manager.

Default: 2400



Specifies the maximum number of Compute nodes that can pre-cache images in parallel.

  • Setting this parameter to a high number can cause slower pre-cache performance and might result in a DDoS on the Image service.
  • Setting this parameter to a low number reduces the load on the Image service, but can cause longer runtime to completion as the pre-cache is performed as a more sequential operation.

Default: 1



Set to True to automatically remove unused base images from the cache at intervals configured by using manager_interval. Images are defined as unused if they have not been accessed during the time specified by using NovaImageCacheTTL.

Default: True



Specifies the minimum age that an unused resized base image must be to be removed from the cache, in seconds. Unused resized base images younger than this will not be removed. Set to undef to disable.

Default: 3600



Specifies the name of the folder where cached images are stored, relative to $instances_path.

Default: _base



Specifies the length of time in seconds that the Compute service should continue caching an image when it is no longer used by any instances on the Compute node. The Compute service deletes images cached on the Compute node that are older than this configured lifetime from the cache directory until they are needed again.

Default: 86400 (24 hours)

4.2. Configuration options for instance ephemeral storage properties

Use the parameters detailed in the following table to configure the performance and security of ephemeral storage used by instances.


Red Hat OpenStack Platform (RHOSP) does not support the LVM image type for instance disks. Therefore, the [libvirt]/volume_clear configuration option, which wipes ephemeral disks when instances are deleted, is not supported because it only applies when the instance disk image type is LVM.

Table 4.2. Compute (nova) service instance ephemeral storage parameters

Configuration methodParameterDescription



Specifies the default format that is used for a new ephemeral volume. Set to one of the following valid values:

  • ext2
  • ext3
  • ext4

The ext4 format provides much faster initialization times than ext3 for new, large disks.

Default: ext4



Set to True to convert non-raw cached base images to raw format. The raw image format uses more space than other image formats, such as qcow2. Non-raw image formats use more CPU for compression. When set to False, the Compute service removes any compression from the base image during compression to avoid CPU bottlenecks. Set to False if you have a system with slow I/O or low available space to reduce input bandwidth.

Default: True



Set to True to use CoW (Copy on Write) images in qcow2 format for instance disks. With CoW, depending on the backing store and host caching, there might be better concurrency achieved by having each instance operate on its own copy.

Set to False to use the raw format. Raw format uses more space for common parts of the disk image.

Default: True



Specifies the preallocation mode for instance disks. Set to one of the following valid values:

  • none - No storage is provisioned at instance start.
  • space - The Compute service fully allocates storage at instance start by running fallocate(1) on the instance disk images. This reduces CPU overhead and file fragmentation, improves I/O performance, and helps guarantee the required disk space.

Default: none

Hieradata override


Set to True to enable direct resizing of the base image by accessing the image over a block device. This is only necessary for images with older versions of cloud-init that cannot resize themselves.

This parameter is not enabled by default because it enables the direct mounting of images which might otherwise be disabled for security reasons.

Default: False

Hieradata override


Specifies the image type to use for instance disks. Set to one of the following valid values:

  • raw
  • qcow2
  • flat
  • rbd
  • default

RHOSP does not support the LVM image type for instance disks.

When set to a valid value other than default the image type supersedes the configuration of use_cow_images. If default is specified, the configuration of use_cow_images determines the image type:

  • If use_cow_images is set to True (default) then the image type is qcow2.
  • If use_cow_images is set to False then the image type is Flat.

The default value is determined by the configuration of NovaEnableRbdBackend:

  • NovaEnableRbdBackend: False

    Default: default

  • NovaEnableRbdBackend: True

    Default: rbd

4.3. Configuring shared instance storage

By default, when you launch an instance, the instance disk is stored as a file in the instance directory, /var/lib/nova/instances. You can configure an NFS storage backend for the Compute service to store these instance files on shared NFS storage.


  • You must be using NFSv4 or later. Red Hat OpenStack Platform (RHOSP) does not support earlier versions of NFS. For more information, see the Red Hat Knowledgebase solution RHOS NFSv4-Only Support Notes.


  1. Log in to the undercloud as the stack user.
  2. Source the stackrc file:

    [stack@director ~]$ source ~/stackrc
  3. Create an environment file to configure shared instance storage, for example, nfs_instance_disk_backend.yaml.
  4. To configure an NFS backend for instance files, add the following configuration to nfs_instance_disk_backend.yaml:

      NovaNfsEnabled: True
      NovaNfsShare: <nfs_share>

    Replace <nfs_share> with the NFS share directory to mount for instance file storage, for example, '' or ''. If using IPv6, use both double and single-quotes, e.g. "'[fdd0::1]:/export/nova'".

  5. Optional: The default mount SELinux context for NFS storage when NFS backend storage is enabled is 'context=system_u:object_r:nova_var_lib_t:s0'. Add the following parameter to amend the mount options for the NFS instance file storage mount point:

      NovaNfsOptions: 'context=system_u:object_r:nova_var_lib_t:s0,<additional_nfs_mount_options>'

    Replace <additional_nfs_mount_options> with a comma-separated list of the mount options you want to use for NFS instance file storage. For more information on the available mount options, see the mount man page:

    $ man 8 mount.
  6. Save the updates to your environment file.
  7. Add your new environment 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/nfs_instance_disk_backend.yaml

4.4. Additional resources