Increasing the number of loop devices

If you run multiple Xen machines, at some point the default number of loop devices might become too small and cause such errors in /var/log/xen/xen-hotplug.log:

 xenstore-read: couldn't read path backend/vbd/23/2051/node

If you have Eucalyptus installed then the symptoms are that some of the newly created instances just terminate shortly after creation. In my case for example, I couldn't get over 2 running instances at one box at once

You can see how many loop devices you have currently by executing the following:

 # ls -l /dev/loop* |wc -l

8 would be the typical answer on Redhat/Fedora. To increase to e.g. 64:

1. Add options loop max_loop=64 to the /etc/modprobe.conf (max value is 256)
2. (in redhat 4/5 and clones) /sbin/rmmod loop ; /sbin/modprobe loop
3. ls -l /dev/loop* |wc -l to check that the num bre of loop devices has increased

On Ubuntu 9.04 Jaunty: In theory (didn't work for me):

1. vi /etc/modprobe.d/local.conf (create or modify)
2. add options loop max_loop=64
3. reboot

In practice:

 ~$ cat loop.sh
 for i in $(seq 8 255); do
        mknod -m0660 /dev/loop$i b 7 $i
        chown root.disk /dev/loop$i
 done