Why it is not possible to rebuild an instance from a bootable volume?

asked 2016-09-29 02:25:54 -0500

viktor-tikkanen gravatar image

When an instance is booted from an image (or volume), it can be rebuilt using different image. In this case e.g. "heat stack-update" command will trigger instance rebuilding if the image is updated into heat template.

But if an instance is booted from a volume (specified in block_device_mapping/block_device_mapping_v2 property either with volume_id or image_id value), after updating the volume/image into heat template "heat stack-update" command will trigger instance replacement (new instance will be launched first and old instance will be terminated after that) even if image_update_policy is explicitly set to REBUILD.

In Horizon GUI the same restriction is visible in Rebuild Instance pop-up window ("Select Image" field is present but it is not possible to specify bootable volume).

It there some reason why an instance cannot be rebuilt from different (bootable) volume (or from different image specified in block_device_mapping_v2)?

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