root@controller:~# nova migrate --poll vm1

Server migrating... 0% complete
Error migrating server
ERROR (InstanceInErrorState): Unexpected error while running command.
Command: ssh 10.10.10.31 mkdir -p /var/lib/nova/instances/63cad586-d3ff-4e4c-a43d-3dbb26a4e128
Exit code: 255
Stdout: u''
Stderr: u"Warning: Permanently added '10.10.10.31' (ECDSA) to the list of known hosts.\r\nPer

i am getting this error please provide solution for this.