Warn if the root filesystem is too small

[bgilbert]

Add a warning if the root filesystem is too small (coreos/fedora-coreos-tracker#586), either because the underlying storage is explicitly specified, or because the root partition is constrained by an additional partition.

[jlebon]

We've decided to defer this until we have coreos/fedora-coreos-tracker#759 because otherwise it's impossible in many cases for Butane to know that it's looking at the root disk. There are some cases where it could (e.g. if root reprovisioning/mirroring is requested), but the common case of just adding a "trapping partition" wouldn't be detectable without some dubious heuristics.

[prestist]

@bgilbert This issue might belong in ignition? Similar to coreos/ignition#1397 (comment)
With this change coreos/fedora-coreos-config#1621

[bgilbert]

Ignition is distro-independent and doesn't know the partition layout constraints of the OS it runs on, so this needs to go in Butane in the fcos variant.

[prestist]

Darn, ok thank you!

[prestist]

@bgilbert

Add a warning if the root filesystem is too small (coreos/fedora-coreos-tracker#586), either because the underlying storage is explicitly specified, or because the root partition is constrained by an additional partition.

I wanted to ask a clarifying question, do we consider root constrained if there is a partition in front of it aka /var on number 5 ?

or is it only constrained if the underlying storage is explicitly specified and there is a partition in front of it ?

In speaking with @jlebon it is fairly common ask to have a /var partition after root for us, and I was unsure if we wanted a warning in a common configuration case.

[bgilbert]

It's common to have a /var partition after the root partition, but we should only warn if that would make the root partition too small. In sum, the values we need to check are:

• The sizeMiB of the root partition, if specified
• The startMiB of any following partition. If this is unspecified (0) and the root sizeMiB is also unspecified, then the following partition will prevent the root partition from growing at all, and we should warn.