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melanie witt 76c10acb33 docs: Show the recommended way to PXE boot an instance

Someone asked about this in the #openstack-nova channel today and I
found some artifacts: an abandoned patch [1] and spec [2] where the
conclusion of the discussion was a recommendation to use iPXE and
rescue to PXE boot an instance.

This adds a small high level doc about how to do it to help others who
may want to do the same thing.

[1] https://review.opendev.org/c/openstack/nova/+/434549
[2] https://review.opendev.org/c/openstack/nova-specs/+/435052

Change-Id: I78f097b9e280fc2bcc14655042b844d3d5977b07

2024-09-06 00:26:09 +00:00

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Boot an instance using PXE

Follow the steps below to boot an existing instance using PXE.

Create an image with iPXE

iPXE is open source boot firmware. See the documentation for more details: https://ipxe.org/docs

Use the iPXE image as a rescue image

Boot the instance from the iPXE image using rescue.

Legacy instance rescue

The ordering of disks is not guaranteed to be consistent.

$ openstack server rescue --image IPXE_IMAGE INSTANCE_NAME

Stable device instance rescue

To preserve the ordering of disks when booting, use stable device rescue.

$ openstack image set --property hw_rescue_device=disk IPXE_IMAGE

or:

$ openstack image set --property hw_rescue_bus=virtio IPXE_IMAGE

or:

$ openstack image set --property hw_rescue_device=disk \
  --property hw_rescue_bus=virtio IPXE_IMAGE

Run the rescue using the API microversion 2.87 or later:

$ openstack --os-compute-api-version 2.87 server rescue \
  --image IPXE_IMAGE INSTANCE_NAME

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