Add BMH boot-iso proposal

design/baremetal-operator/bmh_live_iso.md

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+
+# Add boot-iso API to BareMetalHost
+
+## Status
+
+implementable
+
+## Summary
+
+Add a new interface so that it is possible to boot arbitrary
+iso images via Ironic.
+
+## Motivation
+
+In some circumstances it is desirable to boot an existing iso image
+instead of having Ironic boot IPA, for example:
+
+To reduce boot time for ephemeral workloads, it may be possible to boot an iso
+and not deploy any image to disk (saving the time to write the image and reboot)
+
+Where an alternative installer exists that is distributed as a live-iso, it
+may be desirable to leverage that toolchain instead of the IPA deploy ramdisk,
+for example the [fedora-coreos installer](https://docs.fedoraproject.org/en-US/fedora-coreos/bare-metal/)
+
+An [Ironic API](https://docs.openstack.org/ironic/latest/admin/ramdisk-boot.html)
+exists that supports this but it's not currently accessible via metal3.
+
+### Goals
+
+Expose the Ironic API to boot iso images via metal3.
+
+### Non-Goals
+
+This only considers the ability to boot an iso image (which Ironic supports
+via both redfish-virtualmedia and iPXE), not live kernel/ramdisk images.
+
+Ironic does support booting kernel/ramdisk images though, so the interface
+we decide on shouldn't preclude adding that capability in future.
+
+## Proposal
+
+Add a option to the BareMetalHost DiskFormat spec which indicates that instead
+of deploying an image to disk, a live-iso image will be booted:
+
+```yaml
+  spec:
+    image:
+      url: http://1.2.3.4/image.iso
+      format: live-iso
+    online: true
+```
+
+Note that in this mode, `rootDeviceHints` and `userData` will not be used
+since the image won't be written to disk, and Ironic doesn't
+support passing user-data in addition to the iso attachment at present.
+
+At some future point it would be desirable for Ironic to add the ability
+to pass userData - then a generic iso could be booted with the ability
+to define customization via the userData field in the normal way.
+
+## Design Details
+
+When this mode is selected, we need to configure Ironic to use the [ramdisk
+deploy interface](https://docs.openstack.org/ironic/latest/admin/ramdisk-boot.html)
+which is not currently enabled in ironic-image.
+
+We will also need to write the provided ISO URL into the
+[instance_info boot_iso field](https://docs.openstack.org/ironic/latest/admin/drivers/redfish.html#virtual-media-ramdisk)
+
+### Risks and Mitigations
+
+- Currently there is no detachment API in ironic, so in the case where an installer
+  iso is booted, we don't have a way to detach the virtualmedia and will have to
+  rely e.g on efibootmgr in whatever iso gets booted to ensure the correct boot device.
+
+- The inspection of a BMH will still rely on booting IPA and there's no support for
+  fast-track provisioning in this workflow (since it's booting two different
+  iso images) so this adds a reboot into the process.  This can potentially be
+  avoided in some cases where we can provide a status annotation on creation of
+  the BMH such that inspection is not performed.
+
+- Similarly cleaning of a BMH will rely on IPA, so will require a reboot which may
+  not be desirable in some situations.  This can potentially be avoided if cleaning
+  is [made configurable](https://github.com/metal3-io/metal3-docs/pull/151) in future.
+
+### Work Items
+
+- Enable ramdisk deploy interface in ironic-image
+- Add `live-iso` image format option to the BMH schema
+- Update BMO deploy logic to set the `deploy_interface` and `instance_info` appropriately
+
+### Dependencies
+
+There are some Ironic roadmap items which can be considered soft-dependencies,
+these don't block the initial implementation but may be desirable to make
+this feature more flexible:
+
+[Allow config-drive with virtualmedia ramdisk deploy](https://storyboard.openstack.org/#!/story/2008380)
+
+[Method to detach virtualmedia after initial boot](https://storyboard.openstack.org/#!/story/2008363)
+
+### Test Plan
+
+This should be tested in CI - do we have any metal3 coverage for
+redfish-virtualmedia atm?
+
+### Upgrade / Downgrade Strategy
+
+This new format is added to the BMH API as an optional new interface, all existing
+BMH interfaces should continue to work as before.
+
+On upgrade this new interface will become available, and once in use it will not
+be possible to downgrade, which given the expected use in net-new deployments
+is probably reasonable.
+
+## Alternatives
+
+We could avoid exposing this interface and mandate all users rely on IPA to
+deploy disk images, but this doesn't provide a good solution for the
+emphemeral worload case, and requires testing/supporting two install paths
+where a platform (such as FCOS mentioned) provides existing iso based
+tooling to deploy to disk.