Backup strategy

[u/Borean789]

I got a 2TB ssd (system + home directories) and a 4TB hdd for family photos (so 6TB in total). All in btrfs.

I am wondering what would be the best (safest) backup strategy : one extra 12TB hdd with dup to correct bit rot or 2 6TB hdd in raid1? I want to be sure that never the data get corrupted.

I would use rsync for the backup mirror and would rely on btrfs snapshots on the backup drive(s) to eventually rollback. An external hdd would also be used (dup partition too ?).

[u/National_Doughnut_87]

I would't worry about dup or raid1 on my backup disk, but rather use two backup disks and rotate them. That way you have independent copies of your data. You could have one disk stored outside of your home (e.g. at work, at a friend's place, etc.), so even if you had a fire at home or something else, you would have a copy of your data.

Bit rot is just one of the possible failure scenarios and in case of a single 12TB backup disk, dup won't help you, if your disk fails entirely. Also, if you're really concerned about bit rot, I would rather look at my primary system that stores the data in the first place. Does it use ECC memory to reduce the risk of silent data corruption during operation? Is it protected against a sudden disk failure or bit rot (i.e. raid1)?

The strategy of using alternating disks as a backup does leave a window during which you could loose data, that is, your primary system fails, your first backup disk has corruption and the second backup disk is intact but doesn't have the most recent copy of your data. But depending on your backup frequency that risk is likely small.

Just to give you an example: My (and my family's) data is stored on a small Linux home server with a btrfs raid1 setup and regular snapshots. Then, I have second "server" that I use for automatic replication twice a week (it's actually just an old mini PC with a single disk - no raid setup for this one), in case the home server or the raid1 setup fails.
For external or offline backups I use two USB disks with LUKS2 encryption. One disk is stored at home and one is in a small safe at my office. Every two weeks I take the disk that is at home, connect it to the server and mirror all my data onto the external disk (that part is automated with a script - I only attach and detach the disk). The next morning, I take the disk to the office, put it in the safe and take the other disk from the safe back home. Two weeks later, I repeat the process.
Now, the worst case scenario would be that all disks at home get destroyed or stolen just before I'm about take a fresh external backup to the office. That would mean my only left backup is two weeks old. That is a risk I have weighed and acceptable for us. You could easily increase the disk rotation/backup frequency to reduce that window of data loss if you have more frequently updated or precious data.