---
features:
  - |
    A new configuration option, ``[compute]/max_disk_devices_to_attach``, which
    defaults to ``-1`` (unlimited), has been added and can be used to configure
    the maximum number of disk devices allowed to attach to a single server,
    per compute host. Note that the number of disks supported by a server
    depends on the bus used. For example, the ``ide`` disk bus is limited to 4
    attached devices.

    Usually, disk bus is determined automatically from the device type or disk
    device, and the virtualization type. However, disk bus
    can also be specified via a block device mapping or an image property.
    See the ``disk_bus`` field in
    https://docs.openstack.org/nova/latest/user/block-device-mapping.html
    for more information about specifying disk bus in a block device mapping,
    and see
    https://docs.openstack.org/glance/latest/admin/useful-image-properties.html
    for more information about the ``hw_disk_bus`` image property.

    The configured maximum is enforced during server create, rebuild, evacuate,
    unshelve, live migrate, and attach volume. When the maximum is exceeded
    during server create, rebuild, evacuate, unshelve, or live migrate, the
    server will go into ``ERROR`` state and the server fault message will
    indicate the failure reason. When the maximum is exceeded during a server
    attach volume API operation, the request will fail with a
    ``403 HTTPForbidden`` error.
issues:
  - |
    Operators changing the ``[compute]/max_disk_devices_to_attach`` on a
    compute service that is hosting servers should be aware that it could
    cause rebuilds to fail, if the maximum is decreased lower than the number
    of devices already attached to servers. For example, if server A has 26
    devices attached and an operators changes
    ``[compute]/max_disk_devices_to_attach`` to 20, a request to rebuild server
    A will fail and go into ERROR state because 26 devices are already
    attached and exceed the new configured maximum of 20.

    Operators setting ``[compute]/max_disk_devices_to_attach`` should also be
    aware that during a cold migration, the configured maximum is only enforced
    in-place and the destination is not checked before the move. This means if
    an operator has set a maximum of 26 on compute host A and a maximum of 20
    on compute host B, a cold migration of a server with 26 attached devices
    from compute host A to compute host B will succeed. Then, once the server
    is on compute host B, a subsequent request to rebuild the server will fail
    and go into ERROR state because 26 devices are already attached and exceed
    the configured maximum of 20 on compute host B.

    The configured maximum is not enforced on shelved offloaded servers, as
    they have no compute host.
upgrade:
  - |
    The new configuration option, ``[compute]/max_disk_devices_to_attach``
    defaults to ``-1`` (unlimited). Users of the libvirt driver should be
    advised that the default limit for non-ide disk buses has changed from 26
    to unlimited, upon upgrade to Stein. The ``ide`` disk bus continues to be
    limited to 4 attached devices per server.