Copying copy-on-write file systems without special option can result in copying the merged file system content, i.e., including that of sharing read-only file system. The following link http://uml.openconsultancy.com/#runbackup shows how to back up UML file system.

$ uml_mconsole test stop
$ uml_mconsole test sysrq s
$ cp -fra --sparse=always uml/test/rootfs uml/test/rootfs.bak
$ uml_mconsole test go
$ nice 19 bzip2 uml/test/rootfs.bak

try scp -fra --sparse=always rootfs root@vpc:sis