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Alice
Asked: 2019-01-22 21:41:59 +0800 CST

Why is "loop14" not listed in lsblk?

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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?

command-line loop-device

1 Answers

  1. Best Answer

    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.

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