disk_setup could deal with non-deterministic device naming on EC2 nitro instances

This bug was originally filed in Launchpad as [LP: #1828611](https://bugs.launchpad.net/cloud-init/+bug/1828611)

Launchpad details

affected_projects = []

assignee = None

assignee_name = None

date_closed = None

date_created = 2019-05-10T16:50:27.772455+00:00

date_fix_committed = None

date_fix_released = None

id = 1828611

importance = low

is_complete = False

lp_url = https://bugs.launchpad.net/cloud-init/+bug/1828611

milestone = None

owner = nils-nm

owner_name = Nils Meyer

private = False

status = triaged

submitter = nils-nm

submitter_name = Nils Meyer

tags = []

duplicates = []

_Launchpad user **Nils Meyer(nils-nm)** wrote on 2019-05-10T16:50:27.772455+00:00_

Cloud Provider: Amazon Web Services

As is documented in [1], instances on the nitro type hypervisor don't attach the NVME disks in deterministic order, yielding a different order of disks, example [2]. This makes it somewhat difficult to format and partition volumes since you don't know the volume ids beforehand when creating an instance (in an Autoscaling group for example).

My current thinking is that maybe a sort of special device name (much like swap / ephemeralX) coul be used to locate a device, for example ebs:root for the root drive - which is easy to detect, ebs:size=12G[0] for the first volume found with 12GiB size. With an appropriate instance profile and boto3 more elaborate selectors are conceivable (for example based on tags).

Further complicating things is that the metadata endpoint doesn't expose the correct device names, opting instead for fantasy names (sda1 for the root volume, sdX for other volumes).

My Workaround for a 2 volume instance: Try and format both devices, then mount the disk by label:
#cloud-config
fs_setup:
- label: mylabel
device: /dev/nvme0n1
filesystem: xfs
partition: none
overwrite: false
- label: mylabel
device: /dev/nvme1n1
filesystem: xfs
partition: none
overwrite: false

mounts:
- ["/dev/disk/by-label/mylabel", "/mnt/label", "xfs", "defaults"]

[1] https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/nvme-ebs-volumes.html
[2] https://gist.github.com/nilsmeyer/eddcfa4b7fc5b04ebc0be9eaa3c7b7dd