Dtyjlui

Upon listing the blocks using lsblk, I found 14 look blocks listed, from loop 0 to loop 13.

me@alpha:~$ lsblk

NAME   MAJ:MIN RM   SIZE RO TYPE MOUNTPOINT

loop0    7:0    0  87.9M  1 loop /snap/core/5662

loop1    7:1    0 140.7M  1 loop /snap/gnome-3-26-1604/74

loop2    7:2    0   3.7M  1 loop /snap/gnome-system-monitor/57

loop3    7:3    0 140.9M  1 loop /snap/gnome-3-26-1604/70

loop4    7:4    0  34.6M  1 loop /snap/gtk-common-themes/818

loop5    7:5    0  42.1M  1 loop /snap/gtk-common-themes/701

loop6    7:6    0  89.5M  1 loop /snap/core/6130

loop7    7:7    0   2.3M  1 loop /snap/gnome-calculator/238

loop8    7:8    0 130.2M  1 loop /snap/gnome-3-28-1804/4

loop9    7:9    0  14.5M  1 loop /snap/gnome-logs/45

loop10   7:10   0    13M  1 loop /snap/gnome-characters/124

loop11   7:11   0  53.7M  1 loop /snap/core18/536

loop12   7:12   0    13M  1 loop /snap/gnome-characters/139

loop13   7:13   0   2.3M  1 loop /snap/gnome-calculator/260

sda      8:0    0   113G  0 disk 

├─sda1   8:1    0   512M  0 part /boot/efi

└─sda2   8:2    0 112.5G  0 part /

However, when I list using ls /dev and find blocks using grep 'loop', I found 15 blocks listed, with an extra loop14 shown.

me@alpha:~$ ls /dev | grep 'loop'

loop0

loop1

loop10

loop11

loop12

loop13

loop14

loop2

loop3

loop4

loop5

loop6

loop7

loop8

loop9

loop-control

Why loop14 is not listed from lsblk?

lsblk lists loop devices with a backing file. In your case, loop devices 0-13 have backing files, and show up.

losetup --find, used to find the next available loop device, automatically creates a new loop device if all existing loop devices are in use (if run as root). So some process might have run losetup --find as root to look for an available loop device, and probably didn't use it.

Example:

I have seven loop devices, all backed:

$ sudo losetup --list --all         

NAME       SIZELIMIT OFFSET AUTOCLEAR RO BACK-FILE

/dev/loop1         0      0         1  1 /var/lib/snapd/snaps/core_5897.snap

/dev/loop6         0      0         0  0 /tmp/foo

/dev/loop4         0      0         0  0 /tmp/foo

/dev/loop2         0      0         1  1 /var/lib/snapd/snaps/core_6130.snap

/dev/loop0         0      0         1  1 /var/lib/snapd/snaps/vuze-vs_3.snap

/dev/loop7         0      0         0  0 /tmp/foo

/dev/loop5         0      0         0  0 /tmp/foo

/dev/loop3         0      0         1  1 /var/lib/snapd/snaps/core_6034.snap

$ ls /dev/loop*

/dev/loop0  /dev/loop2  /dev/loop4  /dev/loop6  /dev/loop-control

/dev/loop1  /dev/loop3  /dev/loop5  /dev/loop7

I ask losetup for the next available loop device as root:

$ sudo losetup --find       

/dev/loop8

And losetup has created this for me:

$ ls /dev/loop*             

/dev/loop0  /dev/loop2  /dev/loop4  /dev/loop6  /dev/loop8

/dev/loop1  /dev/loop3  /dev/loop5  /dev/loop7  /dev/loop-control

But it's not in lsblk:

$ lsblk              

NAME   MAJ:MIN RM   SIZE RO TYPE MOUNTPOINT

loop1    7:1    0  88.2M  1 loop /snap/core/5897

loop6    7:6    0     1M  0 loop 

loop4    7:4    0     1M  0 loop 

sr0     11:0    1  1024M  0 rom  

loop2    7:2    0  89.5M  1 loop /snap/core/6130

loop0    7:0    0 280.2M  1 loop /snap/vuze-vs/3

loop7    7:7    0     1M  0 loop 

sda      8:0    0    41G  0 disk 

└─sda1   8:1    0    41G  0 part /

loop5    7:5    0     1M  0 loop 

loop3    7:3    0  89.5M  1 loop /snap/core/6034

Because it's not backed by anything usable as a block device.