Creating multiple servers with a shared volume

Lee Yarwood lyarwood at
Mon Sep 27 08:45:33 UTC 2021

On 25-09-21 13:11:19, open infra wrote:
> On Sat, Sep 25, 2021 at 7:37 AM Laurent Dumont <laurentfdumont at>
> wrote:
> > Just to be sure, the flow is.
> >
> >    - Create VM with root disk on a volume/image.
> >    - Create another 1TB volume.
> >    - Attach the volume to the VM as a second drive using a multi attach
> >    approach (
> >
> >    )?
> >    - Create snapshot of the original VM (with it's root drive + 1 TB
> >    volume)?
> >    - Create new VM from snapshot.
> >    - Instead of a new VM + 1 new root volume + the same 1TB multiattach
> >    volume attached, you get a new multiattach volume (new volume ID and
> >    everything)?

Because you're attaching a snapshot and not the original volume? See
below for more.

> Yes, here is the block device mapping of the snapshot (from a VM with 100GB
> of root disk and 1TB of multiattach volume)
> block_device_mapping[{"image_id": null, "delete_on_termination": true,
> "device_name": "/dev/vda", "disk_bus": "virtio", "volume_id": null,
> "volume_type": null, "destination_type": "volume", "source_type":
> "snapshot", "guest_format": null, "volume_size": 100, "device_type":
> "disk", "snapshot_id": "7b2c7f3a-8420-4a33-820e-2d2231d930c7",
> "boot_index": 0, "no_device": null, "tag": null}, {"image_id": null,
> "delete_on_termination": false, "device_name": "/dev/vdb", "disk_bus":
> null, "volume_id": null, "volume_type": null, "destination_type": "volume",
> "source_type": "snapshot", "guest_format": null, "volume_size": 1000,
> "device_type": null, "snapshot_id": "c2a0695f-8c9e-46f9-80a8-560c47521eeb",
> "boot_index": null, "no_device": null, "tag": null}]

As you can see above you're using source_type = "snapshot" and
destination_type of "volume". As set out in the following docs this
creates a new volume from a given snapshot and attaches that to the

snapshot -> volume - this works exactly as passing type=snap does. It
would create a volume from a Cinder volume snapshot and attach that
volume to the instance. Can be marked bootable.

block_device_mapping_v2.source_type (Optional) 

The source type of the block device. Valid values are:
snapshot: This is only valid with destination_type=volume; creates a
volume backed by the given volume snapshot referenced via the
block_device_mapping_v2.uuid parameter and attaches it to the server

When attaching multiattach enabled volumes to multiple instances you
need to use source_type = 'volume' *and* destination_type = 'volume'.

Hope this helps,

Lee Yarwood                 A5D1 9385 88CB 7E5F BE64  6618 BCA6 6E33 F672 2D76
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