A simple routine for server backups
A backup you have never restored is a hope, not a backup. The routine below is deliberately boring: it copies what matters, records checksums, and gets exercised once a month.
What to copy
Configuration under /etc, service data directories, and a list of installed packages. Everything else can be rebuilt.
rsync -a --delete /etc/ /backup/etc/ dpkg --get-selections > /backup/packages.txt ( cd /backup && find . -type f -print0 | xargs -0 sha256sum > SHA256SUMS )
The restore drill
Once a month, pick one service, restore its configuration into a scratch directory and start it on a spare port. If it comes up, the backup is good. If it does not, you found out on a quiet day.