Re: nextboot warns it won't reset

> On Jul 19, 2024, at 4:45 AM, Gareth de Vaux <stable@lordcow.org> wrote:
> 
> Hi all, nextboot warns as follows:
> 
> # nextboot -k testkernel
> WARNING: loader(8) has only R/O support for ZFS
> nextboot.conf will NOT be reset in case of kernel boot failure
> 
> 
> This's on a ZFS zroot 12.4-STABLE system.

You're encouraged to upgrade to supported releases ;)

> 
> Does this mean that nextboot will not do its job?

I think the WARNING is a good hint and explain why it does not work incase boot failure.

The implementation of load desired kernel is write loader parameters to /boot/nexboot.conf , to indicate
the loader(8) do its job, that is loading the desired kernel on next boot. Well it is a oneshot boot
so the loader will try to disable these parameters by writing `nextboot_enable=NO` to  /boot/nexboot.conf.

That is the magic. Apparently if loader(8) does not support write to filesystem, in this case the ZFS, 
it will fail to write /boot/nexboot.conf in case kernel boot failure. Thus subsequent boot the loader(8) would
still try to boot from the oneshot `testkernel`.

> 
> I see a blog mentioning that nextboot will write metadata to the ZFS pool label,
> in which case maybe the warning is no longer applicable?

That is implementation specific. Normally you can ignore the warning, unless you have trouble booting
the kernel.

> 
> 
> Also I suspect I should rather run "nextboot -D" to revert the situation.
> Is this equivalent to removing /boot/nextboot.conf and/or this metadata in the
> ZFS pool label?

I think normally you do not need that.

Best regards,
Zhenlei