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I've recently been working on a project requiring encrypted files mounted via a connection to a loop device. While the project will run on an Ubuntu system, I was experimenting with setting it up on my Arch Linux workstation. That's when I noticed that Arch doesn't have any loop devices initialized by default, while all the Ubuntu systems I checked (ranging from 18.04 to 22.10) always have 8 pre-created loop devices. This kind of makes sense, since you need elevated privileges to run

losetup -f

On the other hand, the permissions on /dev/loopN don't allow ordinary users to use them anyway. But in any case I can't figure out who/what is creating these devices. I've looked over the udev rules in /lib/udev/rules.d, and couldn't find anything relevant. I also looked through the systemd service files in /lib/systemd/system and couldn't find anything there, either. This is mostly a curiosity question, as I hate magical events in my operating systems.

Here is the evidence that there are no snaps mounted on this system

root@texas-tea:~# ls /dev/loop*
/dev/loop0  /dev/loop2  /dev/loop4  /dev/loop6  /dev/loop-control
/dev/loop1  /dev/loop3  /dev/loop5  /dev/loop7

root@texas-tea:~# lsblk
NAME                          MAJ:MIN RM   SIZE RO TYPE  MOUNTPOINT
sda                             8:0    0 238.5G  0 disk  
├─sda1                          8:1    0   714M  0 part  /boot
├─sda2                          8:2    0     1K  0 part  
└─sda5                          8:5    0 237.8G  0 part  
  └─sda5_crypt                253:0    0 237.8G  0 crypt 
    ├─vgtexas--tea-root       253:1    0  47.7G  0 lvm   /
    ├─vgtexas--tea-swap_1     253:2    0   3.7G  0 lvm   [SWAP]
    ├─vgtexas--tea-var        253:3    0  23.9G  0 lvm   /var
    ├─vgtexas--tea-var+condor 253:4    0   952M  0 lvm   /var/condor
    ├─vgtexas--tea-tmp        253:5    0  11.9G  0 lvm   /tmp
    └─vgtexas--tea-var+local  253:6    0 149.7G  0 lvm   /var/local
sr0                            11:0    1  1024M  0 rom   

root@texas-tea:~# mount | grep loop
root@texas-tea:~# 

root@texas-tea:~# losetup -l
root@texas-tea:~#

EDIT: I recently became aware that loop devices can be created as a compile time option or passed as a kernel parameter (see max_loop= here: https://www.kernel.org/doc/html/v4.14/admin-guide/kernel-parameters.html) Checking /etc/default/[grub|grub.d] I don't see anything related being passed as a kernel parameter on boot, so my current best guess is that the Ubuntu kernel compilers are setting BLK_DEV_LOOP_MIN_COUNT=8 in the build configuration.

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  • Unfortunately, no. In particular, checking the 18.04 machine I use in the office there are the usual 8 loop devices, but losetup -a doesn't show any of them being in use.
    – pgoetz
    Commented Jan 31, 2023 at 23:40
  • Seems like perhaps you missed the point of that duplicate.
    – user535733
    Commented Feb 1, 2023 at 0:16
  • Please explain. It's not a duplicate, or even a related question. There are mounted snaps on the 22.10 machine I checked but none on the 18.04 machine.
    – pgoetz
    Commented Feb 2, 2023 at 1:42
  • The most common explanation by far is the presence of snap packages, hence the duplicate. If there are mysterious loop-mounts without snaps on the 18.04 machine, then edit your question to show appropriate lsblk and df and snapd output. We will need those clues.
    – user535733
    Commented Feb 2, 2023 at 2:00
  • 1
    What was the installation process for setting up this snap-less Ubuntu system? Without knowing how to reproduce this, it's hard to tell you where to look for the cause. As for magical things happening... I have an Arch Linux with % ls /dev/loop* => /dev/loop0 /dev/loop1 /dev/loop2 /dev/loop3 /dev/loop4 /dev/loop5 /dev/loop6 /dev/loop7 /dev/loop-control and without snap installed. I'd guess this is just from the loop module being loaded at all.
    – muru
    Commented Feb 24, 2023 at 8:10

1 Answer 1

Reset to default
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Type lsblk (or even df) and you will see "who/what is creating" these devices :)

These loop devices are mount points for installed snap packages. Each snap package uses one device.

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    No, loop devices are not mountpoints, they are devices that get mounted and you can see their mountpoints (which are usually folders) when you type lsblk. The mounts are created by systemd and you can find the correspondig .mount-files in /etc/systemd/system.
    – mook765
    Commented Jan 31, 2023 at 22:06
  • @mook765 Unfortunately I can't find any .mount files in /etc/systemd/system. find . -depth -iname \*.mount returns nothing.
    – pgoetz
    Commented Jan 31, 2023 at 23:51
  • @raj As pointed out by mook765, loop devices are not mountpoints, nor are they block devices. They don't show up in lslbk unless you bind them to something. For example, if I run losetup /dev/loop0 my-file-to-mount.img loop0 will show up in lsblk. When I run lsblk on the ubuntu systems I'm looking at, no loop devices show up in lsblk indicating they're not bound to anything.
    – pgoetz
    Commented Jan 31, 2023 at 23:54
  • This obviously isn't the case here...
    – Artur Meinild
    Commented Feb 24, 2023 at 14:17

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