Choosing a level in 2026
The honest short version: RAID 10 for databases and anything write-heavy, RAID 6 for bulk storage on large drives, RAID 1 for simple two-disk boot volumes, RAID 0 only for data you can lose without a meeting, and RAID 5 with increasing caution as disk sizes grow. The calculator gives you the capacity arithmetic; the level choice is really about what happens during a rebuild.
The rebuild problem
When a disk fails, the array runs degraded until a replacement is fully rebuilt, and rebuilding a 20 TB drive takes a day or more of reading every sector of every surviving disk. That stress window is exactly when a second failure, or a single unrecoverable read error on an otherwise-healthy disk, is most likely. RAID 5 has no answer to that; RAID 6's second parity exists precisely for it, and RAID 10 sidesteps it by rebuilding via a straight copy from one mirror partner. This is also why hot spares and email alerts on the first failure matter more than the level itself: a degraded array nobody notices for a month is a countdown.
What RAID is not
RAID keeps a service running through disk hardware failure. It does nothing against deletion, ransomware, filesystem corruption, controller failure or a fire, every one of those is faithfully mirrored to all disks in milliseconds. The capacity you "lose" to parity is availability spending, not backup spending; the 3-2-1 backup rule still applies on top. And whatever the level: schedule regular scrubs (echo check > /sys/block/md0/md/sync_action on Linux mdraid) so latent sector errors are found while redundancy still exists to fix them.