RAID levels
RAID is a Redundant Array of Inexpensive (or Independent) Disks: multiple physical disks acting as one logical unit for redundancy, performance, or both. It’s managed in software by the OS or in hardware by a RAID controller.
- RAID 0, striping: survives nothing, any disk loss kills the set. Better reads and writes.
- RAID 1, mirroring: survives 1 disk. Better reads (either copy serves), no write gain since both disks write.
- RAID 4, striping + a dedicated parity disk: survives 1 disk. Better reads, no write gain, because the parity disk is the bottleneck.
- RAID 5, striping + distributed parity: survives 1 disk. Better reads and writes.
- RAID 6, striping + two distributed parity blocks: survives 2 disks. Better reads and writes.
Parity rebuild: after a disk dies, data is recreated from parity, with degraded performance until the failed drive is replaced and rebuilt.
Nested (hybrid) levels: RAID 10 is mirrored pairs striped together, RAID 0+1 is stripes mirrored, RAID 50 is RAID 5 sets striped.
Mixing drives: different types, speeds, and sizes can coexist in one system but never in one RAID group. Mixed capacities in a group waste space; usable size per disk = the smallest disk in the group.