BorgBackup

BookStack organizes documentation into a clear hierarchy: shelves, books, chapters, and pages. It deduplicates, meaning if you back up 100GB and only 2GB changed since yesterday, it only stores the 2GB. Over time, that saves enormous amounts of storage.

Borg also encrypts by default. Your backups are unreadable without your key. Compression is built in (LZ4, zstd, LZMA). It's been around since 2015, battle-tested, and trusted by sysadmins who don't trust much.

Completely free. No paid tier, no cloud service, no strings. You point it at a local drive, a remote server via SSH, or a service like BorgBase ($2/mo for 10GB) or rsync.net ($0.015/GB/mo with Borg support).

The catch: Borg is CLI-only. No GUI, no web dashboard. If you want scheduled backups, you write a cron job or use Borgmatic (a wrapper that handles scheduling and config). The learning curve is real. The first backup setup takes an hour of reading docs. After that, it just works. Also, Borg 2.0 has been in beta for a long time. If that bothers you, Restic is the modern alternative with a similar approach but more active development.