Should disks in a raid really prevent booting?

[Rick Stevens]

On 03/25/2015 04:26 AM, Tom Horsley wrote:
> I was apparently missing something like the raid1 kernel
> module in the initramfs in my fedora 21 partition, but
> I disabled the mdmonitor service and removed the raid
> filesystem from the /etc/fstab, yet the system still
> could not boot. It spent several minutes trying to recognize
> the raided disks then went into the dracut shell.
>
> Should it really be utterly impossible to boot a system
> that merely has raided disks connected to it which no
> one is trying to reference? Should I make a kernel bug
> for this?

I believe grub* will ignore things in /etc/fstab marked as "noauto" as
they aren't to be mounted at boot time so the lack of the md module
wouldn't matter. Things that don't have "noauto" set will be mounted by
"mount -a" at some early time in the multiuser boot sequence. Remember
that fstab doesn't indicate it's a RAID, just a disk and/or partition,
so the system doesn't know to have the RAID module loaded unless it's
in the modprobe stuff.

I don't think the system was unbootable (at least to single-user mode).
Single user mode just needs the kernel running and the root filesystem
mounted. Other run levels will start mounting up things in /etc/fstab
and if it doesn't have "noauto", the system is going to assume it's
needed.

The last part of your question implies that you're asking grub to
predict if some program started at boot time is going to reference that
RAID or not. That's a big ask. So, tag it as "noauto" in fstab and
manually mount it in the equivalent of "rc.local" or ensure that the md
module is in your modprobe configs so grub picks it up on a update to
the grub environment. Don't expect grub to figure out if the disk is
actually being used.
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- Rick Stevens, Systems Engineer, AllDigital    ricks at alldigital.com -
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