What is the difference between RAID levels, and how do you plan storage capacity?

RAID combines physical disks for redundancy, performance, or both.

• RAID 0 (striping) — data split across disks. Fast, full capacity, and no redundancy at all: one disk failing loses everything. Only for scratch data you can recreate.
• RAID 1 (mirroring) — identical copies on two disks. Survives one failure, gives fast reads, but costs 50% of capacity.
• RAID 5 (striping with distributed parity) — survives one disk failure, with the capacity of n-1 disks. The concern is rebuild time: on large modern drives a rebuild takes many hours under full load, and a second failure during it loses the array.
• RAID 6 — dual parity, surviving two simultaneous failures. The sensible choice for large arrays precisely because of the rebuild risk.
• RAID 10 — mirrored pairs, striped. Best performance and fast rebuilds, at 50% capacity cost. Standard for databases.

Capacity planning: baseline current usage, measure the growth rate over several months rather than guessing, project forward, and add headroom — filesystems degrade well before 100%, and 80% is a common trigger point. Plan procurement lead time into the threshold, so the alert fires early enough to actually order hardware.

Note: The essential point: RAID is availability, not backup. It protects against disk failure only — not deletion, corruption, or ransomware, all of which replicate instantly across every disk in the array.