Configuring the Immutable Object Cache

If a compute node has fast local disks (such as NVMe or PMEM), ceph clients such as Nova using the RBD interface for volumes can use these disks as a local read-only cache for volumes created from snapshots, for example, when a snapshot of a Glance image in the ceph cluster is made in order to create a bootable Cinder volume.

The copy-on-write mechanism means that all volumes cloned from the same snapshot on the same compute host can share this cache and may avoid the need to repeatedly read from OSDs the same underlying blocks (i.e those from the original Glance image). New data written to the volume will not be cached with the Immutable Object Cache.

The immutable object cache runs a daemon on the client node and must be an authorised user of the ceph cluster. To enable the immutable object cache on Nova compute nodes, create the following config in /etc/openstack_deploy/group_vars/nova_compute.yml, taking care to consider any other use of ceph_client_ceph_conf_overrides in the deployment as the definition should only appear once.

ceph_immutable_object_cache_enabled: true

    rbd_plugins: parent_cache
    rbd_parent_cache_enabled: true
    immutable_object_cache_path: /ceph-immutable-object-cache
    immutable_object_cache_max_size: 1500G    # set max size appropriate to the cache disk capacity

As part of the pre-deployment configuration, operators must prepare a suitable disk and mountpoint which defaults to /ceph-immutable-object-cache, this can be changed by overriding the ceph_immutable_object_cache_dir variable.

For ceph clusters which are not deployed using OpenStack-Ansible, a keyring must be created for a immutable object cache client:

ceph auth get-or-create client.immutable-object-cache mon 'allow r' osd 'profile rbd-read-only'

When the service is deployed and correctly configured, a cache directory structure will be created inside the cache directory after a new VM is booted using a disk cloned from a Glance image or Cinder snapshot. Existing VM will not be cached.