Migration fails because disk image cannot be found

A domain cannot be migrated because libvirt complains that it cannot access disk image(s):

# virsh migrate qemu qemu+tcp://fedora2/system
error: Unable to allow access for disk path /var/lib/libvirt/images/qemu.img: No such file or directory

Background

By default, migration only transfers in-memory state of a running domain (memory, CPU state, ...). Disk images are not transferred during migration but they need to be accessible at the same path from both hosts.

Solution

Some kind of shared storage needs to be setup and mounted at the same place on both hosts.

The simplest solution is to use NFS:

  • Setup an NFS server on a host serving as shared storage (this may even be one of the hosts involved in migration, as long as all hosts involved are accessing via NFS):

# mkdir -p /exports/images
# cat >>/etc/exports <<EOF
/exports/images    192.168.122.0/24(rw,no_root_squash)
EOF

  • Mount the exported directory at a common place on all hosts running libvirt (let's suppose the IP address of our NFS server is 192.168.122.1):

# cat >>/etc/fstab <<EOF
192.168.122.1:/exports/images  /var/lib/libvirt/images  nfs  auto  0 0
EOF
# mount /var/lib/libvirt/images

Beware, that naive solution of exporting a local directory from one host using NFS and mounting it at the same path on the other host would not work. The directory used for storing disk images has to be mounted from shared storage on both hosts. Otherwise, the domain may lose access to its disk images during migration because source libvirtd may change the owner, permissions, and SELinux labels on the disk images once it successfully migrates the domain to its destination. Libvirt avoids doing such things if it detects that the disk images are mounted from a shared storage.