Nesting LVM for resizeable VMs

As part of my role wearing the DevOps hat at DigiResults I’m in charge of setting up new machines; since I’m terminally lazy I clearly want to automate this. My first attempt used the ubuntu-vm-builder (which is a ubuntu-provided wrapper around the more generic vmbuilder script and was good enough for the first version but not without its drawbacks - namely I got bitten by making the VM disk too small.

vmbuilder will happily write to a ‘raw’ partition, which in my case was a LVM logical volume. The easiest way to show what I mean is by utilising my stunning ASCII-art skills. Running (not the complete command):

ubuntu-vm-builder kvm --raw /dev/mapper/vms-my_vm --hostname=my_vm

produces as disk layout the looks something like this (the PV and VGs aren’t shown as they just complicate matters

--- LVM on Host -------------------
| /dev/mapper/vms/my_vm (LV)      |
|                                 |
|    ----- VM ------------------- |
|    | /dev/sda*                | |
|    |                          | |
|    |                          | |
|    |                          | |
|    |                          | |
|    ---------------------------- |
-----------------------------------

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