#3: Get more loop devices
Loop devices on Linux are virtual devices which can be used to mount files like real devices. For instance to mount an ISO image you can use a loop device.
mkdir -p /mnt/myiso
mount -o loop -t iso9660 ./disk.iso /mnt/myisoThe parameter -o loop tells mount to mount this as a loop device. For each mounted loop device a block device file in /dev is used, named /dev/loopN where N is a serial number, starting at 0. Normally the first free device is used but you can also specify manually which one to use with losetup. To mount an ISO on /dev/loop3 run
losetup /dev/loop3 ./disk.iso
mount -t iso9660 /dev/loop3 /mnt/myisoTo unmount the ISO file and detatch it from /dev/loop3 run
umount /mnt/myiso
losetup -d /dev/loop3For historical reasons you have 8 such loop devices by default, numbered from 0 to 7. If all devices are occupied, you will get the message “mount: could not find any free loop device” when trying to allocate more of them.
In current Kernel releases you can of course use more than just 8 loop devices. To get n loop devices append max_loop=n as parameter to the kernel command line of your bootloader config. Reboot and enjoy!
The second method is to build the kernel configuration directive CONFIG_BLK_DEV_LOOP as a module. You find this under
Device Drivers --->
[*] Block devices --->
<M> Loopback device support
The built module will be named loop. To configure this module append the following line to your modules config (normally /etc/modules.conf or one file in /etc/modprobe.d/):
options loop max_loop=n
Again, n is the number of devices to create. When you're done, reboot. If you have no loop devices mounted right now you can also just reload the module loop instead of rebooting.
All good things come in threes so there's yet another way to get more loop devices. You can create them manually with mknod. To create a ninth device run
mknod -m 660 /dev/loop8 b 7 8The parameter -m specifies the file rights (here: -rw-rw----), /dev/loop8 is the name of the device to create and b stands for Block Device. The numbers at the end are the major and minor numbers. The first one (the major number) sets the device type and therefore which driver to use. Loop devices have the major number 7. The second digit (the minor number), is a number used internally by the driver. This can be any cipher, in this case it's the number of the loop device counted from 0.
As an alternative you can install the script MAKEDEV (yes, it's spelled all caps) which is easier to handle. Just run
MAKEDEV loop from within the /dev directory. Personally I dislike this tool since it lacks a lot of flexibility and control but it's your decision which tool you prefer.
You should note that devices created with either mknod or MAKEDEV do not survive a reboot. If you steadily need more devices try the first or second method.
Incidentally, there's no way of setting the number of loop devices in the kernel config. This number is hardcoded in /linux/drivers/block/loop.c (C code with line numbers):
1618 if (max_loop) {
1619 nr = max_loop;
1620 range = max_loop;
1621 } else {
1622 nr = 8;
1623 range = 1UL << (MINORBITS - part_shift);
1624 }If max_loop is not set, 8 loop devices are created by default (nr ist set to 8 on line 1622). You may hack this by changing that line but that's not a good idea. You'd run into trouble when updating the kernel.
One last thing to mention: loop devices are often also called loopback devices. This is not all wrong, even the kernel config calls it loopback. However, not to mix it up with network loopbacks (localhost, 127.0.0.1, ::1) I prefer the name loop device.
RT @reflinux: #Advent series "24 Short #Linux #Hints", day 3: Get more #loop #devices http://bit.ly/guZcEQ