allow disabling ephemeral disk formatting

Registered by sean mooney

today we support specifying a default format to use for ephemeral disks created by nova.
and we have logic to auto-select based on the guest os in the image

This blueprint tracks enabling a way to opt out of the formating of ephemeral disk so that they are just presented
like cinder volumes as a blank device to the guest. this will enable operators to remove the libguestfs depency form nova with libvirt if they do not use file injection with is already deprecated in nova.

Blueprint information

Status:
Not started
Approver:
Sylvain Bauza
Priority:
Undefined
Drafter:
sean mooney
Direction:
Approved
Assignee:
sean mooney
Definition:
Approved
Series goal:
Accepted for antelope
Implementation:
Unknown
Milestone target:
None

Related branches

Sprints

Whiteboard

there is one open question here shoudl the existing config option be extended
https://docs.openstack.org/nova/latest/configuration/config.html#DEFAULT.default_ephemeral_format to add unformated or a new bool option added to disable formating which defaults to false to keep exsign behavior.

i was originally going to do the former but i think the latter is cleaner code wise.

we discussed this in the nova team meeting on the 17th of January add agreed to proceed with this as a specless blueprint and and a new value "unformatted" to model the desired behavior. additional the help text will be updated to document the behaviour of the current default "None" which means the format is dependent on the virt driver and virt_type.

The motivating usecases are:
- as an operator, i would like to delegate partitioning and formatting of nova ephemeral disks to the end user
- as a packager i would like to reduce the container footprint by making libguestfs optional where ephemeral disk formating is not used and file injection is not required.

(?)

Work Items

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