Why there are 3 loop devices on my system right after boot?

Question

I'll start this question by listing all drives available on my system right after the login into my Linux Mint 19.2 Cinnamon:

# fdisk -l

Disk /dev/loop0: 89,1 MiB, 93454336 bytes, 182528 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes


Disk /dev/loop1: 89,1 MiB, 93429760 bytes, 182480 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes


Disk /dev/loop2: 202,9 MiB, 212713472 bytes, 415456 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes


Disk /dev/nvme0n1: 477 GiB, 512110190592 bytes, 1000215216 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: gpt
Disk identifier: 989573D5-37E7-437A-B680-9410F7234A94

Device          Start        End    Sectors   Size Type
/dev/nvme0n1p1   2048     194559     192512    94M EFI System
/dev/nvme0n1p2 194560 1000214527 1000019968 476,9G Linux filesystem


Disk /dev/sda: 232,9 GiB, 250059350016 bytes, 488397168 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: gpt
Disk identifier: B05FAE90-8E4B-4CCC-AC77-D35D4B260C8A

Device         Start       End   Sectors   Size Type
/dev/sda1       2048   1026047   1024000   500M EFI System
/dev/sda2    1026048   1288191    262144   128M Microsoft reserved
/dev/sda3    1288192 459608774 458320583 218,6G Microsoft basic data
/dev/sda4  459610112 461398015   1787904   873M Windows recovery environment
/dev/sda5  461398016 485992447  24594432  11,7G Windows recovery environment
/dev/sda6  485992448 488396799   2404352   1,2G Windows recovery environment

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Here, I should probably add, I don't even know what a loop device is. I just mount a lot of disks daily, make images of them, work with those and such. I have noticed those loop devices 0, 1, 2 as of this moment, but they might very well be here for ages, maybe I just did not pay attention to them, due to work stress.

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Listing them does not reveal when they originated probably as I see today's date:

# ll /dev/loop[012]

brw-rw---- 1 root disk 7, 0 Nov 13  2019 /dev/loop0
brw-rw---- 1 root disk 7, 1 Nov 13  2019 /dev/loop1
brw-rw---- 1 root disk 7, 2 Nov 13  2019 /dev/loop2

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Why they're on my system even if I reboot? Isn't that strange?

Here is my /etc/fstab:

UUID=f1fc7345-be7a-4c6b-9559-fc6e2d445bfa    /               ext4     errors=remount-ro                0    1
UUID=4966-E925                               /boot/efi       vfat     umask=0077                       0    1
none                                         /ramdisk        tmpfs    size=1G,mode=0777                0    0
UUID=01D480EF7EDA45C0                        /mnt/windows    ntfs     nosuid,nodev,nofail,noexec,rw    0    0

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How do I get more information about these 3 loop devices?

I tried fdisk now:

# fdisk /dev/loop0

Welcome to fdisk (util-linux 2.31.1).
Changes will remain in memory only, until you decide to write them.
Be careful before using the write command.

The old squashfs signature will be removed by a write command.

Device does not contain a recognized partition table.
Created a new DOS disklabel with disk identifier 0xc54bc258.

Command (m for help): v
Remaining 182527 unallocated 512-byte sectors.

Command (m for help): p

Disk /dev/loop0: 89,1 MiB, 93454336 bytes, 182528 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0xc54bc258

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I've already regenerated my initramfs image with no change, they're still there after reboot.

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I'd rather know more about them before actually trying to delete them.

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Questions and answers to comments:

Why are you not as concerned by the various /dev/tty* devices as by the loop devices?

Well, this is a tough question, as I don't know exactly what those tty devices purpose is. So, I guess ignorance is bliss, sort of speak.

Why do you want to delete the /dev/loop* devices specifically?

I said I want to know more about them, to investigate what they contain, well, and as per fdisk output, they do not, correction, contain any partition table. What's more, these loop devices are not present on any other Linux I am directly working with.

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It seems, it has some relation to Snap daemon.

# losetup -a

/dev/loop1: [66306]:6161256 (/var/lib/snapd/snaps/core_8039.snap)
/dev/loop2: [66306]:6160663 (/var/lib/snapd/snaps/core_7917.snap)
/dev/loop0: [66306]:6160545 (/var/lib/snapd/snaps/vlc_1049.snap)

# losetup -l
NAME       SIZELIMIT OFFSET AUTOCLEAR RO BACK-FILE                           DIO LOG-SEC
/dev/loop1         0      0         1  1 /var/lib/snapd/snaps/core_8039.snap   0     512
/dev/loop2         0      0         1  1 /var/lib/snapd/snaps/core_7917.snap   0     512
/dev/loop0         0      0         1  1 /var/lib/snapd/snaps/vlc_1049.snap    0     512

Answer 1

The existence of /dev/loop* devices doesn’t necessarily indicate anything; some systems pre-populate them, others don’t. However, if they show up in fdisk -l’s output, that means they’ve been set up to provide a loop device over either a file or another block device.

You can see what loop devices are set up, and what they “loop” over, by running

losetup -l

as root. In your case, the devices are being used to access Snap volumes;

snap list

will list the running Snaps on your system (apparently VLC and the core Snap). See the Ubuntu Snap tutorial for a brief introduction.