A device can be upgraded to a later Ubuntu Core release if it has:
- an assigned serial number: from either a serial assertion or the Snap Store.
- identical storage layout: layouts are identical unless they’ve been manually reconfigured.
- consistent base and system snaps: significant differences will cause an upgrade to fail.
It is not currently possible to upgrade from releases earlier than UC20, or from either a UC20 reference image or a reference model assertion, including those available on https://cdimage.ubuntu.com/ubuntu-core/20. However, reference model assertions can be easily modified to build a custom image that can be upgraded.
Upgrades are performed by remodelling, which is the process of replacing one model assertion with another on a deployed system. For an upgrade from UC20 to UC22, for example, this means updating the base and system snaps in a UC20 model assertion with their UC22 counterparts.
Prerequisites
On the device, run snap model to make sure the device has been assigned a serial:
$ snap model
brand <brand-id>
model ubuntu-core-20-amd64
grade signed
serial 810d53c7-a741-9351-0a7e2b96095b
The contents of /etc/os-release show which version of Ubuntu Core is running:
$ cat /etc/os-release
NAME="Ubuntu Core"
VERSION="20"
ID=ubuntu-core
PRETTY_NAME="Ubuntu Core 20"
VERSION_ID="20"
HOME_URL="https://snapcraft.io/"
BUG_REPORT_URL="https://bugs.launchpad.net/snappy/"
Model assertions
The model assertion defines which system snaps make up the device’s operating system, including the gadget snap, kernel snap and the base snap with the (read-only) root filesystem.
On an Ubuntu Core 20 system, kernel and gadget snaps are installed from a 20 track, with the base snap being core20. The snapd snap remains unchanged, as it’s always at the latest version available.
The following is a complete unsigned reference model for an amd64 Ubuntu Core 20 image:
{
"type": "model",
"series": "16",
"authority-id": "<your-brand-id>",
"brand-id": "<your-brand-id>",
"model": "ubuntu-core-20-amd64",
"architecture": "amd64",
"serial-authority": ["generic"],
"timestamp": "2021-10-05T09:50:53+00:00",
"base": "core20",
"grade": "signed",
"snaps": [
{
"name": "pc",
"type": "gadget",
"default-channel": "20/stable",
"id": "UqFziVZDHLSyO3TqSWgNBoAdHbLI4dAH"
},
{
"name": "pc-kernel",
"type": "kernel",
"default-channel": "20/stable",
"id": "pYVQrBcKmBa0mZ4CCN7ExT6jH8rY1hza"
},
{
"name": "core20",
"type": "base",
"default-channel": "latest/stable",
"id": "DLqre5XGLbDqg9jPtiAhRRjDuPVa5X1q"
},
{
"name": "snapd",
"type": "snapd",
"default-channel": "latest/stable",
"id": "PMrrV4ml8uWuEUDBT8dSGnKUYbevVhc4"
}
]
}
The above example includes the necessary serial-authority definition to ensure the device is given a serial number. In this case, ["generic"] will ask for a serial to be generated and assigned by the Snap Store.
To create a model assertion that can be used to upgrade a device running on an image generated with the above example, replace the 20 tracks with 22 and core20 with core22 for the base snap:
{
"type": "model",
"series": "16",
"authority-id": "<your-brand-id>",
"brand-id": "<your-brand-id>",
"model": "ubuntu-core-22-amd64",
"architecture": "amd64",
"revision": "1",
"serial-authority": ["generic"],
"timestamp": "2021-10-05T09:50:53+00:00",
"base": "core22",
"grade": "signed",
"snaps": [
{
"name": "pc",
"type": "gadget",
"default-channel": "22/stable",
"id": "UqFziVZDHLSyO3TqSWgNBoAdHbLI4dAH"
},
{
"name": "pc-kernel",
"type": "kernel",
"default-channel": "22/stable",
"id": "pYVQrBcKmBa0mZ4CCN7ExT6jH8rY1hza"
},
{
"name": "core22",
"type": "base",
"default-channel": "latest/stable",
"id": "amcUKQILKXHHTlmSa7NMdnXSx02dNeeT"
},
{
"name": "snapd",
"type": "snapd",
"default-channel": "latest/stable",
"id": "PMrrV4ml8uWuEUDBT8dSGnKUYbevVhc4"
}
]
}
The above example has an updated model name, ubuntu-core-22-amd64, but this arbitrary and can remain the same or be something entirely different.
Note that with the exception of the base snap, the snap id for each snap also remain the same, as does the brand-id of the Snap Store account responsible for creating the original image.
Another important requirement for the updated model is the inclusion of a revision field.
The presence of a revision value permits an assertion to be updated to an updated assertion with a higher revision value. If no revision is specified, as in our previous UC20 example, a revision value of 0 is implied. Consequently, our UC22 example above has a revision value of 1. See Assertions for further details.
The updated model will need to be signed with the same key that the original model was signed with. See Signing a model assertion for detailed instructions.
Reference unsigned model assertions for both UC20 and UC22 can be found here: https://github.com/snapcore/models.
Perform the upgrade
To manually upgrade a device running the original UC20 model assertion, first copy the signed and updated UC22 model assertion to the device. This can be accomplished in many ways, but scp OpenSSH secure file copy is a good option:
scp <uc22-model.model-assertion> <sso-account-name>@<device-ip>:/home/<sso-account-name>
The final step is to run sudo snap remodel <uc22-model.model-assertion> on the device itself:
$ ssh <sso-account-name>@<device-ip>
<ubuntu-core> $ sudo snap remodel my-uc22-model.model-assertion
Download snap "pc-kernel" (991) from channel "22/beta"
[...]
The process will take some time and will involve several device restarts before eventually completing. You can then reconnect and check it’s been upgraded to UC22:
$ cat /etc/os-release
NAME="Ubuntu Core"
VERSION="22"
ID=ubuntu-core
PRETTY_NAME="Ubuntu Core 22"
VERSION_ID="22"
HOME_URL="https://snapcraft.io/"
BUG_REPORT_URL="https://bugs.launchpad.net/snappy/"
And also using the new model assertion:
$ snap model
brand <brand-id>
model ubuntu-core-22-amd64
grade signed
serial 810d53c7-a741-9351-0a7e2b96095b