---
features:
  - |
    The libvirt driver now allows utilizing file backed memory for qemu/KVM
    virtual machines, via a new configuration attribute
    ``[libvirt]/file_backed_memory``, defaulting to 0 (disabled).

    ``[libvirt]/file_backed_memory`` specifies the available capacity in MiB
    for file backed memory, at the directory configured for
    ``memory_backing_dir`` in libvirt's ``qemu.conf``. When enabled, the
    libvirt driver will report the configured value for the total memory
    capacity of the node, and will report used memory as the sum of all
    configured guest memory.

    Live migrations from nodes not compatible with file backed memory to nodes
    with file backed memory is not allowed, and will result in an error. It's
    recommended to upgrade all nodes before enabling file backed memory.

    Note that file backed memory is not compatible with hugepages, and is not
    compatible with memory overcommit. If file backed memory is enabled,
    ``ram_allocation_ratio`` must be configured to ``1.0``

    For more details, see the admin guide documentation:

    https://docs.openstack.org/nova/latest/admin/file-backed-memory.html