rdiff-backup versions 0.2.x require Python version 2.1 or later, and versions 0.3.x require Python version 2.2 or later. If you don't know what version of python you are running, type in "python -V" from the shell. I'm sorry if this is inconvenient, but rdiff-backup uses generators, iterators, and nested scoping extensively, and these were only added in version 2.2.
There is no formal specification, but here is a rough description (settings are always cumulative, so 5 displays everything 4 does):
| 0 | No information given |
| 1 | Fatal Errors displayed |
| 2 | Warnings |
| 3 | Important messages, and maybe later some global statistics (default) |
| 4 | Some global settings, miscellaneous messages |
| 5 | Mentions which files were changed |
| 6 | More information on each file processed |
| 7 | More information on various things |
| 8 | All logging is dated |
| 9 | Details on which objects are moving across the connection |