Loopback devices are insufficient when guest is started on XEN.

Symptom:

Generally, when you attach multiple img instances to a guest instance, you may find that you cannot create a guest instance or start a guest instance.

The error in/var/log/xen/xen-hot-plug.log is as follows:

Xenstore-read: couldn't read path backend/vbd/12/2050/node

 

Cause:

Max_loop = 32 can be seen in the kernel of 32: xen by default.

When there are many guest and each has a lot of disks and other img mounted, it is easy to use it.

Check the number of loopback devices: # ls/dev/| grep loop | wc-l

Number in use: # losetup-

 

Solution:

Set max_loop to a larger value.

Loopback devices are insufficient when guest is started on XEN.