doc/cloud-init: add a note that recent cloud-init on older Ubuntu still doesn't support modern keys

[simondeziel]

No description provided.

[simondeziel]

@holmanb I couldn't find where I read it before but I think that cloud-init backports to old releases (like 20.04) intentionally disable support for the LXD datasource and causes LXD guests to use the nocloud one. Is that right?

$ lxc init ubuntu-minimal-daily:20.04 f1

$ lxc config set f1 cloud-init.user-data - << EOF
#cloud-config
runcmd:
  - touch /root/cloud-init.user-data
EOF

$ lxc config set f1 user.user-data - << EOF
#cloud-config
runcmd:
  - touch /root/user.user-data
EOF

$ lxc start f1 && lxc exec f1 -- cloud-init status --wait
$ lxc exec f1 -- cloud-init status --long | grep detail
detail: DataSourceNoCloud [seed=/var/lib/cloud/seed/nocloud-net]
$ lxc exec f1 -- cloud-init query userdata
#cloud-config
runcmd:
  - touch /root/user.user-data

$ lxc exec f1 -- ls -l /root/
total 1
-rw-r--r-- 1 root root 0 Dec 18 23:27 user.user-data

$ lxc exec f1 -- dpkg -l | grep cloud-init
ii  cloud-init                     24.3.1-0ubuntu0~20.04.1           all          initialization and customization tool for cloud instances

In the above, we see that only the legacy key user.user-data was taken into account.

[simondeziel]

For comparison, here is with a more modern Ubuntu version (22.04):

$ lxc init ubuntu-minimal-daily:22.04 j1

$ lxc config set j1 cloud-init.user-data - << EOF
#cloud-config
runcmd:
  - touch /root/cloud-init.user-data
EOF

$ lxc config set j1 user.user-data - << EOF
#cloud-config
runcmd:
  - touch /root/user.user-data
EOF

$ lxc start j1 && lxc exec j1 -- cloud-init status --wait
$ lxc exec j1 -- cloud-init status --long | grep detail
detail: DataSourceLXD


$ lxc exec j1 -- cloud-init query userdata
#cloud-config
runcmd:
  - touch /root/cloud-init.user-data


$ lxc exec j1 -- ls -l /root/
total 1
-rw-r--r-- 1 root root 0 Dec 18 23:31 cloud-init.user-data

$ lxc exec j1 -- dpkg -l | grep cloud-init
ii  cloud-init                      24.3.1-0ubuntu0~22.04.1                 all          initialization and customization tool for cloud instances

[holmanb]

@holmanb I couldn't find where I read it before but I think that cloud-init backports to old releases (like 20.04) intentionally disable support for the LXD datasource and causes LXD guests to use the nocloud one. Is that right?

Yes. Thanks for this @simondeziel!