Rotating back-ups, removed files, etc.

[Craig Hartnett <craig@1811.spamslip.com>]

Hi again,

OK, so I did read that I'm supposed to forget everything I know about
back-ups, but frankly that wasn't much. :) Not that I know nothing, but
it hasn't been something I've spent a *lot* of time thinking about.

But as I think about Tarsnap, deleted files, rotating/deleting archives,
daily storage charges (increasing, of course, as the amount of data
stored slowly increases), etc., I start wondering about what happens to
files I intentionally delete from my hard drive. If I understand Tarsnap
correctly, a file that I backed up in my initial back-up and that hasn't
since changed only exists in that initial back-up archive because (a) it
hasn't changed so there has been no need to re-upload any part of it and
(b) archives are immutable. If I delete that initial archive I assume (I
could be wrong, so this is part of my series of questions) that Tarsnap
will realise that and back up those files again. Am I right?

So if I delete my initial archive today, Tarsnap will realise that it
has to upload pretty much everything -- not everything, but almost
everything -- again, right?

And what if I delete a file -- any file -- on my hard drive that has
been backed up in the past? Of course Tarsnap won't upload a null file,
but does that file continue to exist in the archives unless or until I
delete the last archive that contains it? In other words, it's *my*
responsibility to curate my archives, right? (I'm quite happy to curate
my own stuff. Just want to make sure.)

And what if I want to delete a file from my hard drive *and* my
back-ups? Since the archives are immutable, and this file was in my
initial back-up, am I right that there is no way to delete that single
file from the back-up archives without deleting the whole archive, and
consequently re-uploading most of the original archive again?

Which leads me to the conclusion that I should pick a time frame -- say,
90 days -- or come up with some traditional, staggered rotation system,
and start deleting archives older than that *except* the initial
archive, right?

Or am I completely out to lunch here? :)

Thanks for any light you can shed on this, via links to documentation
that covers it of course if I have missed it.


Craig