---
fixes:
  - |
    Since Libvirt v.1.12.0 and the introduction of the `libvirt issue`_ ,
    there is a fact that if we set cache mode whose write semantic is not
    O_DIRECT (i.e. "unsafe", "writeback" or "writethrough"), there will
    be a problem with the volume drivers (i.e. LibvirtISCSIVolumeDriver,
    LibvirtNFSVolumeDriver and so on), which designate native io explicitly.

    When the driver_cache (default is none) has been configured as neither
    "none" nor "directsync", the libvirt driver will ensure the driver_io
    to be "threads" to avoid an instance spawning failure.

    .. _`libvirt issue`: https://bugzilla.redhat.com/show_bug.cgi?id=1086704
