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What are these empty (0B SIZE) read-write loop devices?

sudo lsblk --all

NAME   MAJ:MIN RM  SIZE RO TYPE MOUNTPOINTS
loop0    7:0    0    0B  0 loop 
loop1    7:1    0    0B  0 loop 
loop2    7:2    0    0B  0 loop 
loop3    7:3    0    0B  0 loop 
loop4    7:4    0    0B  0 loop 
loop5    7:5    0    0B  0 loop 
loop6    7:6    0    0B  0 loop 
loop7    7:7    0    0B  0 loop 
sda      8:0    0  100G  0 disk 
└─sda1   8:1    0  100G  0 part /

sudo mount | grep -i loop
sudo cat /proc/mounts | grep -i loop
sudo cat /etc/fstab | grep loop
sudo lsof /dev/loop0

No output.

sudo dmesg | grep loop

[    0.572944] Calibrating delay loop (skipped), value calculated using timer frequency.. 4390.12 BogoMIPS (lpj=8780240)
[   15.902123] systemd[1]: Starting modprobe@loop.service - Load Kernel Module loop...
[   15.909809] loop: module loaded
[   15.915496] systemd[1]: modprobe@loop.service: Deactivated successfully.
[   15.915643] systemd[1]: Finished modprobe@loop.service - Load Kernel Module loop.
  • This never happened in Debian 11 (bullseye).
  • This was introduced in Debian 12 (bookworm).

How are these loop devices created? What is the purpose of these loop devices?

adrelanos
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1 Answers1

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These are loop devices, created by the loop module. They are used to present any underlying storage as a block device; typically, this is used to mount a file system stored in an image.

A few of them are instantiated by default, but they remain empty until they are associated with underlying storage. lsblk doesn’t show empty devices, lsblk --all does.

This behaviour isn’t directly related to the switch from Debian 11 to 12; it happens on Debian 11 too, if the loop module is loaded.

Stephen Kitt
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  • How many you get depends on the max_loop module parameter (see /sys/module/loop/parameters/max_loop for the value which can be changed at loading time or as kernel cmdline parameter when the module is builtin, not run time) – Stéphane Chazelas Aug 24 '23 at 18:51
  • The modprobe@loop.service seems to be dependency of systemd-repart.service – Stéphane Chazelas Aug 24 '23 at 18:55