Consider replacing the `linear` array redundancy level in the docs

[hrismarin]

Currently, the Butane and Ignition docs state:

• level (string): the redundancy level of the array (e.g. linear, raid1, raid5, etc.).

Some md personalities, including linear, have been deprecated in the Linux kernel, see:
Ignition fails at raid creation
torvalds/linux@608f52e
torvalds/linux@849d18e

Should we consider removing or replacing the explicit mention of the linear array redundancy level from the Butane and Ignition docs?

[travier]

That's a good point. If the configs are not longer accepted by the kernel and haven't been for a while then butane should probably not accept them as well.

Removing them from the docs is a good first step. Adding some logic in butane or ignition to fail validation in a second step would be good as well. We can probably start in Butane as that could be considered a breaking change in Ignition so not sure if we can do that in a regular spec bump.

[hrismarin]

md-linear was reintroduced, see torvalds/linux@127186c.

I tested fedora-coreos-42.20250204.91.0-qemu.x86_64.qcow2.xz image which has kernel-6.14.0-0.rc0.20250130git72deda0abee6.11.fc42.x86_64 with linear array redundancy level in the Butane config and it was applied successfully.

So when kernel version 6.14 lands in the FCOS stable stream, perhaps we can close this issue.

[jlebon]

See also coreos/ignition#1803