I know if you put 20 IT people in a room to discuss backup strategies, you'd likely get 20 different sets of requirements, but I know for some people the equivalent of "Differential" backups is a potentially useful feature and I think I may have almost stumbled upon a pseudo-version of such a feature (or, at least, the makings of) when I spotted some unexpected behaviour entirely of my own doing via a silly config mistake.
I just noticed that a (test) tape job was writing more than I was expecting it to and upon closer examination I found that it was writing the same .vib files as I already saw it writing yesterday.
When I looked into it I spotted that I'd rather stupidly configured the Media Pool (incremental-specific in this case) to "Do not protect data (cyclically overwrite tapes as required)". I probably did it for a reason originally but that reason now escapes me :-/
Once I realised what was going on I set a more appropriate protection option for the pool but whilst doing so it occurred to me that what I was almost effectively getting was the equivalent of a "Differential" backup of the incrementals. The obvious flaw in this was that, although it was re-writing the growing set of increments, the act of doing so was also overwriting the only version of those files on tape, but I figured that it shouldn't be a massive stretch to build in some logic to emulate this process, whilst actually not
overwriting the tape.
Perhaps it's equivalent to "back-up latest chain as differential" whereby the most recent full, plus it's subsequent incrementals, will always be on the same tape.
I dunno, maybe I'm talking daft ideas but it struck me as an interesting scenario so I thought I'd at least share my madness