lvm misdetected: as root=dm-2; should be dm-0 or rhel7-root

[Mustafa Kocaturk]

This is a problem that requires editing the grub.cfg after every update as follows:

diff -r1.2 /boot/grub/grub.cfg
320c320
< linux /vmlinuz-0-rescue-094290ad3c25419d9b671b141b6ea616 root=/dev/dm-0
---
> linux /vmlinuz-0-rescue-094290ad3c25419d9b671b141b6ea616 root=/dev/dm-2
333c333
< linux /vmlinuz-0-rescue-094290ad3c25419d9b671b141b6ea616 root=/dev/dm-0
---
> linux /vmlinuz-0-rescue-094290ad3c25419d9b671b141b6ea616 root=/dev/dm-2
345c345
< linux /vmlinuz-3.10.0-862.3.2.el7.x86_64 root=/dev/dm-0
---
> linux /vmlinuz-3.10.0-862.3.2.el7.x86_64 root=/dev/dm-2

The target kernel expects to find the root filesystem on dm-0, also known as rhel_smt2-root, not on dm-2.
However, Ubuntu kernel numbers the dm-* devices differently, as follows:

mu@mu-HP-Z620-Workstation:~/Desktop Fri Aug 28 11:39:11 1 1 $ sudo dmsetup ls
200828_113958
rhel_smt2-swap (253:4)
rhel_smt2-root (253:2)
vgubuntu-root (253:0)
vgubuntu-swap_1 (253:1)
rhel_smt2-home (253:3)
mu@mu-HP-Z620-Workstation:~/Desktop Fri Aug 28 11:39:58 1 0 $ ls -l /dev/dm-*
200828_114013
brw-rw---- 1 root disk 253, 0 Aug 28 10:30 /dev/dm-0
brw-rw---- 1 root disk 253, 1 Aug 28 10:30 /dev/dm-1
brw-rw---- 1 root disk 253, 2 Aug 28 10:30 /dev/dm-2
brw-rw---- 1 root disk 253, 3 Aug 28 10:30 /dev/dm-3
brw-rw---- 1 root disk 253, 4 Aug 28 10:30 /dev/dm-4

That is, the dm- minor device numbering differs.

Symptom: The boot menuentry breaks whenever os-update is called, practically after every new kernel update, and rhel7 becomes unbootable.

Suggested workaround: Use the OS-defined name, rhel_smt2-root, instead of dm-2 in the root= parameter. Reverse lookup of minor device number in the first table gives the OS name.