From a high level persepective, using a RAID of external disks as a backup device ...
... has the following benefits:
- Logically, you only have to backup data once (the RAID layer handles the redundancy when you copy data)
- Some configurations can detect bit-rot and auto-correct it (btrfs-raid, md-raid + dm-integrity)
... and the following disadvantages:
- If one of the disks is not present (e.g. if you forgot to plug in one of the data or power cables), you're unable to cleanly assemble the RAID device
- If one of the disks fails, or is disconnected during operation for whichever reason, you have to rebuild the RAID device
- If the filesystem is faulty, all disks contain faulty data, because the faulty data is replicated by the RAID layer (true for md-raid, lvm-raid; false for btrfs-raid, zfs-raid) - If, for example, you would use a md(adm)-RAID-1 with a btrfs-filesystem ontop of it, and the next kernel update (which includes the btrfs code) comes with a bug in the btrfs code, and this bug corrupts btrfs-filesystems, both disks would contain a valid md-RAID-1 device with a corrupted btrfs-filesystem ontop.
- If the RAID-layer code contains a bug, both disks are corrupted, too - The same argumentation as for filesystem-bugs applies
My advice is to not use a RAID of multiple external disks as a backup device and to instead use the disks independently with independent filesystems and execute your backup solution serially for each one of them.
IMHO, RAID should be used to provide high-availability. Backup needs redundancy, and this includes redundancy on a filesystem level (multiple separate filesystem instances).
My advice is to create independent filesystems on each one of your external disks and backup your data with a (e.g.
rsync) script to each one of them. You can run multiple instances of the script in parallel (one per disk) to speed up the backup process...
I am looking to learn the setup for RAID
A good way to do this is using a (e.g.
kvm) virtual machine. This allows for creating as many virtual disks as you want to experiment with.