Howto create Loop Device on Linux

This mini howto explain how to create more loop device on your operating system.You need to create more loop device in some case like encrypted file system or HVM xen virtualization.  By default there are 7 devices available, named /dev/loop0 through to /dev/loop7.

INFORMATIONS
You can find the loop device like that :

1	# ls -al /dev/loop*

2	brw-r----- 1 root disk 7, 0 Mar 19 09:34 /dev/loop0

3	brw-r----- 1 root disk 7, 1 Mar 19 09:34 /dev/loop1

4	brw-r----- 1 root disk 7, 2 Mar 19 09:34 /dev/loop2

5	brw-r----- 1 root disk 7, 3 Mar 19 09:34 /dev/loop3

6	brw-r----- 1 root disk 7, 4 Mar 19 09:34 /dev/loop4

7	brw-r----- 1 root disk 7, 5 Mar 19 09:34 /dev/loop5

8	brw-r----- 1 root disk 7, 6 Mar 19 09:34 /dev/loop6

9	brw-r----- 1 root disk 7, 7 Mar 19 09:34 /dev/loop7

The creation of the loop device is very simple but you have 2 options for this creation one is without reboot and the other one is with a reboot.


HOT CREATION
Creation of loop8 :

1	# mknod -m640 /dev/loop8 b 7 8

2	-m640 define the permission of the device

3	/dev/loop8 define the name of the device

4	b for the creation of the special block device

5	7 8 the number 7 AND 8 define the MAJOR AND the MINOR

Check if the loop is created

1	# ls -l /dev/loop8

2	brw-r----- 1 root root 7, 8 Oct 3 14:54 /dev/loop8

Now you have the applied the good owner on this device

1	#chown root:disk /dev/loop8

Now you have to change the module limitation and reboot your system.
STANDARD CREATION
You have to modify this file /etc/udev/makedev.d/50-udev.nodes

01	#vim /etc/udev/makedev.d/50-udev.nodes

02

loop0

03

loop1

04

loop2

05

loop3

06

loop4

07

loop5

08

loop6

09

loop7

10

loop8

11

parport0

12

parport1

13

parport2

14

parport3

15

net/tun

16

ppp

17

console

18

null

19

zero

Now you have to change the module limitation and reboot your system.

MODULE LIMITATION
In the kernel, you have to allow more creation of loop device.
There is an absolute maximum of 256 devices limited in the kernel. The default limit is set to 8.

01	# modinfo loop

02	filename: /lib/modules/2.6.18-53.1.14.el5.centos.plusxen/kernel/drivers/block/loop.ko

03	alias: block-major-7-*

04	license: GPL

05	srcversion: 82DB6DB3DABF3B945D6394D

06

depends:

07	vermagic: 2.6.18-53.1.14.el5.centos.plusxen SMP mod_unload gcc-4.1

08	parm: max_loop:Maximum number of loop devices (1-256) (int)

09	module_sig: 883f35047d15787f0a982eee333b2d8112f51f09f68f09b79223a34983b3aef66820c08

10

11	cc6e8cd3709f6d7c56171e5a579ee1ca2ca56349c56b983a7c46

Now you have to change this value with the modprobe.conf file.

1	#vim /etc/modprobe.conf

2

...

3	options loop max_loop=64

4

...

TIPS & COMMAND
The commande line losetup is used for set up and control loop device.

1

# losetup

2

usage:

3	losetup loop_device # give info

4	losetup -d loop_device # delete

5	losetup -f # find unused

6	losetup -a # list all used

7	losetup -r # read-only loop

8	losetup [-e encryption] [-o offset] [-r] {-f|loop_device} file # setup