Add LVM as a disk backend

[skylt]

Enables the use of Logical Volumes as disk backends.
This was needed as we store our VMs through LVM and wanted to build
the image directly onto a lv.

It uses an existing volume group and has no support for creating a new one.
It will not override an existing logical volume and fail gracefully.

The lv is created, activated and then mounted as a loop device.
The boostraping process is then launched on the loop device.
Once the process is completed, the lv is unmounted and desactivated.

The created lv will be deleted should the boostraping process fail.

The lv must be activated before use.

A manifest has been included for testing purposes.

[andsens]

Wow, this is awesome. Great adherence to the guidelines and all the right parts of the API are used :-)
I can definitely see this expanding into supporting creation of a VG on the other volume backings at some point, allowing you to boot any machine directly from a LV :-D

I have added some comments that would make the addition a little more robust and expand it so it can support providers other than kvm.

[andsens]

Wouldn't it be better to just pass vg and lv directly on to create() and generate the image_path there? Then you don't need to do the path splitting in _before_create().

[andsens]

Instead of only supporting this on kvm, you could modify volume_group in task_groups.py to be a function and add the commands there. The same would actually work for the loopback tasks as well.

[skylt]

Which providers do you have in mind ?

[andsens]

Come to think of it, there might not be any. This is pretty kvm specific, since the volume is kind of tied to the host machine, correct?
I'm ok with merging this if you squash your commits :-)

[skylt]

The commits have been squashed.

[andsens]

Wouldn't lvm be more accurate?

[andsens]

I don't think there's any need to put vg and lv under another key. Just spell them out directly in volume. So: volume: {logivalvolume: "lvname", volumegroup: "vgname"}

[andsens]

For better manifest validation the volume object should be split into a standardvolume definition and logicalvolume definition, like so:

  volume:
    type: object
    oneOf:
      - $ref: '#/definitions/standardvolume'
      - $ref: '#/definitions/logicalvolume'
definitions:
  ...
  standardvolume:
    type: object
    properties:
      backing: {type: string}
      partitions:
        type: object
        oneOf:
          - $ref: '#/definitions/no_partitions'
          - $ref: '#/definitions/partition_table'
    required: [partitions]
    additionalProperties: false
  logicalvolume:
    type: object
    properties:
      backing: lvm
      volumegroup: {type: string}
      logicalvolume: {type: string}
      partitions:
        type: object
        oneOf:
          - $ref: '#/definitions/no_partitions'
          - $ref: '#/definitions/partition_table'
    required: [volumegroup, logicalvolume, partitions]
    additionalProperties: false

[skylt]

I have updated the various issues.