[Linux] psutil.disk_partitions() returns /dev/root on Ubuntu 20.04 on AWS

[albertvaka]

Summary

• OS: Ubuntu 20.04
• Architecture: 64bit
• Psutil version: 5.8.0
• Python version: 3.8.10
• Type: core

Description

On a default AWS instance running Ubuntu 20.04, psutil.disk_partitions() returns /dev/root instead of /dev/xvda1.

This is because /proc/mounts contain this entry:

/dev/root / ext4 rw,relatime,discard 0 0

However, the actual root disk device is /dev/xvda1. Both mount and findmnt do return the right device:

$ mount
/dev/xvda1 on / type ext4 (rw,relatime,discard)
[...]

$ findmnt -n -o SOURCE /
/dev/xvda1

I found some people doing a workaround to call findmnt manually in this case, but I think this should be handled by psutil.

[giampaolo]

https://unix.stackexchange.com/questions/295060/why-on-some-linux-systems-does-the-root-filesystem-appear-as-dev-root-instead

In here it says that /dev/root is a link. I guess that if we readlink() it we'll get the real device name? Can you try?

[giampaolo]

Actually it seems it's more complicated than that: https://unix.stackexchange.com/questions/17563/find-out-what-device-dev-root-represents-in-linux/431968

[giampaolo]

This should do it:

import os

def get_fs_device(path):
    pmap = {}
    with open("/proc/partitions") as f:
        for line in f.readlines()[2:]:
            fields = line.split()
            major = int(fields[0])
            minor = int(fields[1])
            name = fields[3]
            pmap[(major, minor)] = name

    dev = os.stat(path).st_dev
    name = pmap[(os.major(dev), os.minor(dev))]
    return "/dev/%s" % name

print(get_fs_device("/"))

On my Linux it prints /dev/nvme0n1p2.

[albertvaka]

I can confirm this does return the right device for me (/dev/xvda1 in this case). Also that /dev/root is not a link for me.

Is this something that psutil.disk_partitions() should do before returning the devices, or is this something I should do on my end after calling psutil.disk_partitions()?

[giampaolo]

Something like this will eventually have to be included in psutil, but in case of error we should probably return "/dev/root" instead of crashing.

Also, I see there are multiple variants of the above code online (read /sys fs, use readlink(), parse /proc/cmdline, ...). We will probably have to implement these variants, and the code should try them all before (silently) giving up.

[giampaolo]

Fixed in #2000. Out of curiosity, can you please paste the output of ls -l /dev/root?

[albertvaka]

That was awesomely fast, thanks a ton!

Here you have the output you requested:

$ ls -l /dev/root
brw------- 1 root root 202, 1 Oct  8 14:04 /dev/root

And the same on the actual disk:

$ ls -l /dev/xvda1 
brw-rw---- 1 root disk 202, 1 Oct  8 14:04 /dev/xvda1

[giampaolo]

What about readlink -f /dev/root?

[albertvaka]

$ readlink -f /dev/root
/dev/root