The old format have many issues and is discouraged by tar developers. In
this case the most important one is header with possible non-ASCII
characters, which will result in UnicodeDecodeError (tarfile module
require header parts in utf-8).
PAX format is much cleaner, as it use standard mechanism for extended
headers.
tar can't write archive with _contents_ of block device. We need this to
backup LVM-based disk images. To avoid dumping image to a file first,
create a simple tar archiver just for this purpose.
Python is not the fastest possible technology, it's 3 times slower than
equivalent written in C. But it's much easier to read, much less
error-prone, and still process 1GB image under 1s (CPU time, leaving
along actual disk reads). So, it's acceptable.
