[Discuss] Running things from initramfs

[me at mattgillen.net (Matthew Gillen)]

On 3/5/2021 9:07 PM, Dale R. Worley wrote:
> Here's a messy problem.  I suspect the answer is simple, but obscure:
> 
> I have an Oracle Linux (a Red Hat derivative) computer and I'd like to
> run "xfs_repair /dev/mapper/ol-root".  The problem of course
> is that partition is the root partition and xfs_repair can't operate
> on a partition that's mounted.  If I could boot off a CD, it would be
> simple, but since this computer is remote, that's hard to do.  I've recently
> found that you can edit the "kernel boot line" to include
> "rdinit=/bin/bash", and that causes the boot process to stop early,
> with only the initramfs mounted and you talking to a Bash shell.
> 
> The problem is that this environment is not at all set up, and even
> /dev is nearly empty.  There is an xfs_repair binary, so I figure if I
> could just get /dev/mapper/ol-root set up correctly, I could
> repair it.  There is an "lvm" binary, and it seems to support the
> usual commands, but I can't get it to set up /dev/mapper.
> 
> When I run "lvm" it complains about (IIRC) /proc and /sys not
> containing what it wants.  I've discovered that I can do "mount -t
> proc non /proc" and similarly for /sys and those seem to work.  That
> stops "lvm" from complaining but it doesn't cause it to populate
> /dev/mapper.
> 

Dale,
You could try the rescue target by adding this to your kernel command
line instead of changing the rdinit:
 systemd.unit=rescue.target
or if that mounts your root fs and does the chroot, try emergency mode:
 systemd.unit=emergency.target

If your initramfs contains systemd, then try just starting the
local-fs.target (try "systemctl start local-fs.target").  That might set
everything up that you need.  With luck it doesn't do the chroot to your
LVM/XFS root filesystem.

If that doesn't work, this might give you some hints about using systemd
to mount the right stuff:
https://www.golinuxcloud.com/mount-filesystem-without-fstab-systemd-rhel-8/

Finally, if you get frustrated and feel like you trust your backups, the
option is called dangerous, but one thing you could try is boot to
single-user mode, then remount the root filesystem read-only:
 mount -o remount,ro /

Then do the dangerous repair on a mounted filesystem:
 xfs_repair -d /dev/mapper/ol-root

Then reboot.

HTH,
Matt