nova-specs/specs/newton/approved/flavor-root-disk-none.rst

Add support for flavors with no local ephemeral disks

https://blueprints.launchpad.net/nova/+spec/flavor-root-disk-none

This feature adds the possibility to define a flavor, that does not create any ephemeral disk locally on the hypervisor. The proposal is to add a new flavor key local_disks. If local_disks is set to False, no hypervisor-local root disk or any other local ephemeral disk will be created. A bootable volume needs to be created to launch an instance from that flavor. All other additional disks can only be volume-based, as well.

Problem description

Currently there is no way to force root disk being on cinder and having no hypervisor-local ephemeral disks at all. Setting root_gb to 0 defaults in creating an ephemeral root disk with the size of the glance image.

We do not want to have ephemeral disks locally on the hypervisor; not on a local filesystem nor on a share mounted to /var/lib/nova/instances. As these shares grow, they become unmaintainable and performance gets worse. Storing the ephemeral root disk in the hypervisor's local filesystem is also not a good idea, since consumer data on the root disk often cannot simply be deleted. Having more or less all block devices in cinder volumes gives us a unique and flexible feature set for maintaining block storage of instances and the instances themselves.

Use Cases

The use case for End Users is to not create ephemeral (root) disks locally on the hypervisor by accident. When launching an instance from a flavor with disabled local disks, the user will be informed to create a bootable volume in order to launch the instance with that volume. The volume can be an existing one or a new one, created upon the instance launch request via the block-device parameter. The end user does not need to worry about disk placement; which impacts instance migration, data persistence/loss, and/or shared storage performance.

The use case for deployers is to have the ability to prevent customers from creating ephemeral (root) disks locally on the hypervisor. Deployers do not have to worry to much about local HV filesystem sizing nor they do not need to create one big share for /var/lib/nova/instances, which gets mounted on all hypervisors. As that share grows it gets unmaintainable and performance is gets worse. They do not have to deal with customer data loss on ephemeral disks, stored on local hypervisor FS, in case of hypervisor outage.

Proposed change

The scope of the change:

• new flavor key local_disks in the API
• new boolean column local_disks for table flavors
• default value for local_disks is True
• older microversions display value of 0 for root_gb, ephemeral_gb and swap_gb, if local_disks=False
• newer microversions do not return the keys root_gb, ephemeral_gb and swap_gb, if local_disks=False
• if local_disks=False is given root_gb, ephemeral_gb and swap_gb are optional
• if local_disks=False is given root_gb, ephemeral_gb and swap_gb are automatically set to 0, given values get ignored
• if local_disks is omitted or set to True, root_gb is mandatory again
• error, when flavor create request with local_disks=False and root_gb > 0 and/or emphemeral_gb > 0 and/or swap_gb > 0
• return descriptive exception, when flavor with local_disks=False and requested image larger than given blockdevice layout
• return descriptive exception, when flavor with local_disks=False and given block_device parameter includes dest_type 'local'
• return descriptive exception, when resizing instance from flavor with local_disks=True to flavor with local_disks=False
• adjust api doc to include detailed description of local_disks key
• config-drive will not be touched by that change, config-drive will still reside on local HV disks, like libvirt.xml and console.log do

Alternatives

The alternative is to always boot instances with the additional block_device parameter and dest_type 'volume', to make sure (root) disks are on cinder volumes and no hypervisor-local ephemeral disk gets created. But this has the downside, that someone might forget it and ephemeral disks get created locally on the HV. There is no way to force users, that no ephemeral root disks are being created on the hypervisor.

Data model impact

• add new column local_disks type boolean to flavor table (default is True)

REST API impact

• introduce new api microversion which allows 'None' as disk value
• POST v2.1/<tenant_id>/flavors:

{
    "flavor": {
        "name": "flavor_without_local_ephemeral_disks",
        "ram": 1024,
        "vcpus": 2,
        "local_disks": false,
    }
}

• flavor show omits root_gb, ephemeral_gb and swap_gb, when local_disks=False:

{
    "flavor": {
        "OS-FLV-DISABLED:disabled": false,
        "local_disks": false,
        "os-flavor-access:is_public": true,
        "id": "1",
        "links": [
            {
                "href": "http://openstack.example.com/v2.1/openstack/flavors/1",
                "rel": "self"
            },
            {
                "href": "http://openstack.example.com/openstack/flavors/1",
                "rel": "bookmark"
            }
        ],
        "name": "m1.tiny",
        "ram": 512,
        "vcpus": 1
    }
}

• with older microversions, root_gb, ephemeral_gb and swap_gb will be returned with value 0, when local_disks=False:

{
    "flavor": {
        "OS-FLV-DISABLED:disabled": false,
        "local_disks": false,
        "disk": 0,
        "OS-FLV-EXT-DATA:ephemeral": 0,
        "os-flavor-access:is_public": true,
        "id": "1",
        "links": [
            {
                "href": "http://openstack.example.com/v2.1/openstack/flavors/1",
                "rel": "self"
            },
            {
                "href": "http://openstack.example.com/openstack/flavors/1",
                "rel": "bookmark"
            }
        ],
        "name": "m1.tiny",
        "ram": 512,
        "swap": 0,
        "vcpus": 1
    }
}

• root_gb is no longer mandatory with local_disks=False
• return 400 error when flavor has local_disks=False and no BD mapping given
• return 400 error when flavor has local_disks=False and dest_type local given in BD mapping
• return 400 error when flavor has local_disks=True and instance resize to flavor local_disks=False requested
• return 400 error on flavor create, when local_disks=False and root_gb and/or ephemeral_gb and/or swap_gb given

Security impact

None

Notifications impact

None

Other end user impact

End users need to provide a proper blockdevice mapping with dest_type volume, in order to use a flavor with local_disks=False.

Performance Impact

None

Other deployer impact

If a deployer doesn't want any ephemeral/local disk on the hypervisor nodes, they just create flavors with local_disks=False and then all users of that cloud have to provide a blockdevice mapping with dest_type 'volume' when creating an instance.

Developer impact

None

Implementation

Assignee(s)

Primary assignee:

tpatzig

Work Items

• create db column
• create api microversion with new key
• support local_disks=False for flavor show
• exception handling if local_disks=False in flavor and request contains local BD mapping
• adjust flavor unit test

Dependencies

None

Testing

• Create flavor with local_disks=False
• Boot instance with such flavor without volume
• Boot instance with such flavor with local BD mapping
• Boot instance with such flavor with volume

Documentation Impact

http://docs.openstack.org/openstack-ops/content/flavors.html

API doc will be updated to include the new flavor option local_disks.

References

None

History

Revisions

Release Name	Description
Newton	Introduced