A loopback device is used to access filesystems that are not associated with a block device (Hard Disk drives and CD-ROM drives are examples of block devices). By default there are 8 devices available, named /dev/loop0 through to /dev/loop7.
Extending allowed number of loopbac devices
For CentOS/RHEL 5
To set a limit of “N” loopback devices edit the /etc/modules.conf file, and add the below line to it.
# vi /etc/modules.conf
options loop max_loop=N
When the loop module is loaded or when the system is rebooted, the new setting will take effect. There is an absolute maximum of 256 devices in CentOS/RHEL 5. So you can not exceed the loopback devices beyond that.
For CentOS/RHEL 6
For RHEL6, which uses loopback support directly as part of the kernel, the kernel option ‘max_loop’ can be used. This option can be configured in /etc/grub.conf and is used after a reboot of the system. To set the limit to “N” devices, edit the /etc/grub.conf file, and add max_loop=64 at the end of kernel line.
# vi /etc/grub.conf
...
kernel /vmlinuz-2.6.32-131.0.15.el6.x86_64 ro root=/dev/mapper/root rhgb quiet max_loop=64
initrd /initramfs-2.6.32-131.0.15.el6.x86_64.img
Creating additional loopback device ‘nodes’
Apart from the above 2 ways, you can always create loopback devices using the “mknod” command. Follow the steps outlined belwo to create new loopback devices on the fly.
1. List the existing loopback devices.
# ls -l /dev/loop* brw-r----- 1 root disk 7, 0 Mar 24 17:49 /dev/loop0 brw-r----- 1 root disk 7, 1 Mar 24 17:49 /dev/loop1 brw-r----- 1 root disk 7, 2 Mar 24 17:49 /dev/loop2 brw-r----- 1 root disk 7, 3 Mar 24 17:49 /dev/loop3 brw-r----- 1 root disk 7, 4 Mar 24 17:49 /dev/loop4 brw-r----- 1 root disk 7, 5 Mar 24 17:49 /dev/loop5 brw-r----- 1 root disk 7, 6 Mar 24 17:49 /dev/loop6 brw-r----- 1 root disk 7, 7 Mar 24 17:49 /dev/loop7
2. Create a new device node with the mknod command:
# mknod -m660 /dev/loop8 b 7 8
3. Verify the device node has been created:
# ls -lrt /dev/loop8 brw-rw----. 1 root root 7, 8 Apr 1 01:45 /dev/loop8
4. Set the ownership correctly and verify the ownership:
# chown root.disk /dev/loop8
# ls -lrt /dev/loop8 brw-rw----. 1 root disk 7, 8 Apr 1 01:45 /dev/loop8
