Design for adding UUID and name to devices

This is a design document about adding UUID and name to instance devices (Disks/NICs) and the ability to reference them by those identifiers.

Current state and shortcomings

Currently, the only way to refer to a device (Disk/NIC) is by its index inside the VM (e.g. gnt-instance modify –disk 2:remove).

Using indices as identifiers has the drawback that addition/removal of a device results in changing the identifiers(indices) of other devices and makes the net effect of commands depend on their strict ordering. A device reference is not absolute, meaning an external entity controlling Ganeti, e.g., over RAPI, cannot keep permanent identifiers for referring to devices, nor can it have more than one outstanding commands, since their order of execution is not guaranteed.

Proposed Changes

To be able to reference a device in a unique way, we propose to extend Disks and NICs by assigning to them a UUID and a name. The UUID will be assigned by Ganeti upon creation, while the name will be an optional user parameter. Renaming a device will also be supported.

Commands (e.g. gnt-instance modify) will be able to reference each device by its index, UUID, or name. To be able to refer to devices by name, we must guarantee that device names are unique. Unlike other objects (instances, networks, nodegroups, etc.), NIC and Disk objects will not have unique names across the cluster, since they are still not independent entities, but rather part of the instance object. This makes global uniqueness of names hard to achieve at this point. Instead their names will be unique at instance level.

Apart from unique device names, we must also guarantee that a device name can not be the UUID of another device. Also, to remove ambiguity while supporting both indices and names as identifiers, we forbid purely numeric device names.

Implementation Details

Modify OpInstanceSetParams to accept not only indexes, but also device names and UUIDs. So, the accepted NIC and disk modifications will have the following format:


where, from now on, identifier can be an index (-1 for the last device), UUID, or name and action should be add, modify, or remove.

Configuration Changes

Disk and NIC config objects get two extra slots:

  • uuid
  • name

Instance Queries

We will extend the query mechanism to expose names and UUIDs of NICs and Disks.

Hook Variables

We will expose the name of NICs and Disks to the hook environment of instance-related operations: