[pve-devel] [RFC PATCH 1/2] install: btrfs: fix raid level falling back to single mode

[Christoph Heiss]

On Fri Jan 10, 2025 at 6:00 PM CET, Daniel Kral wrote:
> From a user's perspective, the BTRFS single mode has been removed since
> d85180e ("tui: rename raid levels 0/1 to align with GUI installer"). The
> user can now only select at least the "RAID0" level, but if a user
> selects any raid level with only one disk configured on the system,
> BTRFS will be setup in 'single' mode instead of the chosen raid level.
>
> The TUI installer has a separate check for this, but the GUI installer
> as well as the auto installer will silently fallback to single mode,
> which could be confusing for and unwanted by the user.
>
> Therefore, remove the BTRFS single mode from being selected when
> configuring disks in the GUI installer and during the installation in
> general, which makes the auto installer fail if the wrong amount of
> disks are selected for the specified raid level. This makes btrfs' raid
> disk count validation align with the one from zfs.
>
> Signed-off-by: Daniel Kral <d.kral at proxmox.com>

Tested this patch with GUI, TUI and auto installer, whether they all now
properly reject invalid configurations for each RAID mode. I.e. for
RAID0 <1 disks, RAID1 <2 disks and RAID10 <4 disks. Tested each also
with exactly one disk, of course.

Please consider this patch:

Tested-by: Christoph Heiss <c.heiss at proxmox.com>
Reviewed-by: Christoph Heiss <c.heiss at proxmox.com>

FWIW and completeness: `proxmox-auto-install-assistant validate-answer`
does not (yet) check the filesystem configuration, but the
auto-installer will still fail on invalid configuration as expected.
Michael is working on improving the former, tho.

> ---
>
> Discussion
>
> BTRFS didn't allow a single-disk RAID0 configuration before kernel 5.15
> and AFAIK also silently used the single profile for that case, but this
> has changed since then.
>
> If we want users to still be able to create a BTRFS filesystem in single
> mode (which seems very reasonable), I can do a v2 or followup to add a
> "btrfs (single)" entry to the disk setup.

I think it's fine as it currently is, i.e. we don't really need the
extra single mode.
Since it is supported to create single-disk RAID0 setups
and doesn't result in a degraded state or similar, it is nice to have
a) the simplicity of having one entry/filesystem type less and
b) being consistent with the ZFS side of things on that matter.