[Borgbackup] backing up borg with borg

[Thomas Waldmann]

This 2 stage approach only moves the hard decision from the 1st stage
server to the 2nd stage server.

As you noted, the borg solution for this is append-only mode, but it has
this fundamental and non-trivial problem about having to know when it is
safe to switch off append-only mode and run prune / realize all deletes
/ prunes that have already been issued. And that you have to know
somehow whether the remaining stuff is in a good state and still has all
you need.

When you want to solve this by keeping some history on your 2nd stage
server, you will have to decide the same thing there. And if you can't
decide it on stage 1, you won't be able to decide it on stage 2 either.

Also, you might have some unwanted error propagation from 1st to 2nd
stage (which is why the FAQ rather recommends doing backups to different
places in a parallel, not serial, manner).

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