in reply to Perl's pearls
So when you do a backup, you compare the MD5 signature of each file to see if it match signature of the files already stored.
For each workstation, you also need to save the path of each file and all the meta information (ACLs, major-minor for devices...). Files in the same directory when sorted alphabetically share a "prefix string" so you can store only what changes from one file to the next; example:
/var/log/meesages /var/log/messages.1 # add .1 /var/log/meesages.2 # replace "1" by "2"
But you are lazy and know this have been done before anc check out the source of slocate/updatedb. :)
I delibaretely have overlooked many details. I just wanted to give an example where unique signatures could be used.