Launch Instance - Source - Explained

Openstack allow you to create a virtual machine using four different boot sources:

  • Image

  • Instance snapshot

  • Volume

  • Volume snapshot

By clicking on the “Launch instance” button in Horizon dashboard and choosing “Source” tab you can determine a particular option.

 

Select Boot Source → Choose from Image, Instance Snapshot, Volume and Volume snapshot

Create New Volume → Determine if you need a real/ephemeral root disk

Volume size (GB) (optional) → Set Volume Size if in “Create new Volume” you have chosen “Yes”

Delete Volume on Instance Delete (optional) → Determine if you need to keep volume after Instance termination. It is strongly recommended to set default option “No”.

 


Boot from Image

First option:

Instance booted from Image with real root disk. Openstack uses image stored in Glance reposistory to create a new volume that will become new boot source for instance.

Instance does not contain a Image Name.

It has got a real volume attached on /dev/vda.

The same information you may obtain in Volumes tab.

Second option:

Instance booted from Image with ephemeral root disk. Instance does not contain a real manageable disk. Its snapshots are being seen as VM images and are not related to any disk.

Instance contains a Image Name.

In metadata of virtual machine you may find an image entry.

 


Boot from Instance Snapshot

First option:

Instance booted from Instance snapshot with real root disk. Openstack uses snapshot stored in Glance reposistory to create a new volume that will become new boot source for instance.

Snapshots are being created by clicking on the “create snapshot” button in running virtual machine.

Instance does not contain a Image Name.

It has got a real volume attached on /dev/vda. As you may it almost react in the same way as option “boot from Image”.

If you investigate Volume details, you are able to find out the Snapshot name.

Second option:

Instance booted from Instance snapshot with ephemeral root disk. Instance does not contain a real manageable disk.

Image Name is not empty, it points on the the Snapshot name.

 

In metadata it identifies itself with a snapshot.

 


Boot from Volume

Instance booted from Volume. It does not use Glance in virtual machine creation.

Attention! Bootable Volume has to be prepared beforehand.

It does not have any Image pointed in Instance preview. (To check that, you have to “dig” in volume metadata)

Volume is being attached as a new VM with a newly created volume.

 


Boot from Volume snapshot

Instance booted from Volume snapshot. It uses snapshot of the running volume in time. Its presence is being stored in Cinder.

It does not contain any Image information. It reacts in the same way as volume.

In volume attached we can realised, that Openstack created a new volume based on Volume snapshot.