[excerpt from bup web site]

Bup is a Very efficient backup system based on the git packfile format,
providing fast incremental saves and global deduplication (among and within
files, including virtual machine images).

Bup has a few advantages over other backup software:

-It uses a rolling checksum algorithm (similar to rsync) to split large files
into chunks. The most useful result of this is you can backup huge virtual
machine (VM) disk images, databases, and XML files incrementally, even though
they're typically all in one huge file, and not use tons of disk space for
multiple versions.

-It uses the packfile format from git (the open source version control system),
so you can access the stored data even if you don't like bup's user interface.