Comment 6 for bug 1436314

It is enough to specify --boot-volume option, (see https://wiki.openstack.org/wiki/BlockDeviceConfig for more details about the block device mapping syntax).

Setting max_local_block_devices to 0 means that any request that attempts to create a local disk will fail. This option is meant to limit the number of local discs (so root local disc that is the result of --image being used, and any other ephemeral and swap disks).

AFAIK Tempest by it's very nature will test both booting instances from volumes and from images downloaded to hypervisor local storage, so it makes very little sense to me to attempt to limit the environment tempest runs against to allow only boot from volume and then expect to be able to run tests that spawn instances from images.

 max_local_block_devices set to 0 does not mean that nova will automatically convert --images to volumes and boot instances from volumes - it just means that all request that attempt to create a local disk will fail.