How do you determine the root device if /dev/root is missing?

Question

On the distribution that I am using right now, Arch Linux ARM, Linux is launched without an initial RAM disk, meaning the kernel does all the work in mounting the root filesystem. This system is installed on an SD card and it has a script on it that needs to determine the device node of the filesystem that it resides on. This device node can change depending on how I boot the device up (e.g., I attach it to another device that is already running).

I am able to determine the mount point using df -k --output=target ${0} | sed "1d". To determine the source device, I run df -k --output=source ${0} | sed "1d". If I've booted into Arch Linux, the result of that last command spits out /dev/root, which does not exist. Sure, on some systems, udev will create a symbolic link from /dev/root to the real root device, but mine doesn't do that. I could look at the contents of /proc/cmdline but that stays the same throughout a kernel's uptime, so that means it isn't reliable.

When I get /dev/root, how can I determine the real underlying block device it stands in place for?

Answer 1

Answer from the comments here is very simple. Just call:

findmnt -n -o SOURCE /

Provided that findmnt is present...

Answer 2

The mount command will show it:

mount | sed -n 's|^/dev/\(.*\) on / .*|\1|p'

Answer 3

I found this blog post very helpful in finding root device and primary disk in Linux.

You'll need lsblk utility installed, which is provided by util-linux in CentOS/RHEL. For example, such a variant will give you the disk name of the root device:

lsblk -oMOUNTPOINT,PKNAME -rn | awk '$1 ~ /^\/$/ { print $2 }'
sda3

findmnt from the same package will give you actual underlying root device name:

findmnt -oTARGET,SOURCE -rn /
/ /dev/mapper/vg01-lv_root

Flag -P for both utilities will provide you the same information in the format, suitable for eval in the shell:

findmnt -oTARGET,SOURCE -Pn /
TARGET="/" SOURCE="/dev/mapper/vg01-lv_root"

lsblk -oMOUNTPOINT,PKNAME -Pn | grep 'MOUNTPOINT="/"'
MOUNTPOINT="/" PKNAME="sda3"

Answer 4

lsblk --noheadings --paths --output PKNAME $( findmnt --noheadings --output SOURCE --mountpoint / )

Example output:

/dev/sda

First, you use findmnt to find a device mounted as /, which is /dev/sda2 in my case. Then, you find its parent device using lsblk.

Answer 5

You can use stat / and inspect the device major/minor number or with the help of the rdev command get this interpreted automatically.

Answer 6

Assuming there's a file at /etc/fstab

sudo cat /etc/fstab | grep '\s\/\s'

will give you an fstab entry to the block device mounted on /.

Answer 7

Kernel logs

Do a:

dmesg

First, it might be explicitly given on the kernel CLI at the very start of boot:

Command line: root=/dev/vda ...

Otherwise, at the end of boot, if rootfs is a disk and not initrd / initramfs, the kernel says:

VFS: Mounted root (ext2 filesystem) on device 254:0.

so we then find that:

find /proc /sys -iname '*254:0*'

and then determine what it actually maps through with:

ls -l /sys/dev/block/254:0

output:

lrwxrwxrwx    1 root     root             0 Jan 30 15:59 /sys/dev/block/254:0 -> ../../devices/pci0000:00/0000:00:09.0/virtio4/block/vda

Tested with this QEMU + Buildroot setup.

Answer 8

If you have mounted /sys, then you can extract the real root device from it. The basic idea can be seen in this shell one-liner, if you can grok it:

eval `stat / | awk '/Device:/ { $A=strtonum(substr($2,index($2,"/")+1)); printf "grep DEVNAME /sys/dev/block/%d:%d/uevent\n", int($A/256), $a%256; }'`

Call stat /, then extract the decimal device value and convert it into major and minor numbers using awk, then grep the DEVNAME field from the file /sys/dev/block/<major>:<minor>/uevent. Example output:

# Generic Linux PC
DEVNAME=sda1
# Raspberry Pi
DEVNAME=mmcblk0p2

This works with gawk, but not with mawk. I'm open to suggestions how to make it compatible with both.

UPDATE: I've added a quick Python version also:

import os                                                                       
                                                                                
# Fetch major and minor numbers of root directory.                                                
st = os.stat("/")                                                              
major = os.major(st.st_dev)                                                     
minor = os.minor(st.st_dev)                                                     
# Search for device name in "/sys" filesystem.                                  
with open(f"/sys/dev/block/{major}:{minor}/uevent") as f:                       
    for line in f:                                                              
        key, value = line.rstrip().split("=")                                   
        if key == "DEVNAME":                                                    
            print(value)                                                        
            break                                                               
    else:                                                                       
        print("root device not found")                                          

Answer 9

In my case I have LVM and LUKS setup which looks like the following.

$ lsblk -oNAME,MOUNTPOINT -J
{
   "blockdevices": [
      {"name":"sda", "mountpoint":null,
         "children": [
            {"name":"sda1", "mountpoint":"/boot/efi"},
            {"name":"sda2", "mountpoint":"/boot"},
            {"name":"sda3", "mountpoint":null,
               "children": [
                  {"name":"sda3_crypt", "mountpoint":null,
                     "children": [
                        {"name":"vgmint-root", "mountpoint":"/"},
                        {"name":"vgmint-swap_1", "mountpoint":"[SWAP]"}
                     ]
                  }
               ]
            }
         ]
      },
      {"name":"zram0", "mountpoint":"[SWAP]"},
      {"name":"zram1", "mountpoint":"[SWAP]"},
      {"name":"zram2", "mountpoint":"[SWAP]"},
      {"name":"zram3", "mountpoint":"[SWAP]"}
   ]
}

I want to know that / is on sda. As lsblk can produce JSON with a bit of jq-fu I can be retrieved like this.

$ lsblk -oNAME,MOUNTPOINT -J | \
  jq -r '.blockdevices[] | . as $bd | .. | objects | select(.mountpoint == "/") | $bd.name'
sda