Networking / storage
RAID, NAS, SAN, backups
RAID 0/1/5/10, NAS vs SAN, and the 3-2-1 backup rule.
RAID configurationsOften missed
| Level | Min disks | Idea | Fault tolerance | Rebuild access |
|---|---|---|---|---|
| RAID 0 | 2 | Stripe | None | N/A — any disk loss kills the array |
| RAID 1 | 2 | Mirror | 1 disk | Survives one failed mirror member |
| RAID 5 | 3 | Stripe + parity | 1 disk | Often cannot use volume until rebuild finishes |
| RAID 10 | 4 | Mirror + stripe | 1 per mirror pair | Can often stay online during rebuild |
RAID 5: blocked during rebuild on this study guide. RAID 10: accessible during rebuild because of mirrored pairs.
- Hardware RAID uses a controller card; software/virtual RAID is OS-based. Servers often have onboard RAID; desktops may need a card.
- RAID presents disks as one logical drive. RAID is not a backup.
NAS, SAN, 3-2-1
- NAS: Ethernet (often 1–2.5 Gb), sometimes USB. File sharing and media for home/SMB. Ethernet can bottleneck. WD common; Synology can lock to vendor drives.
- SAN: high-speed enterprise (Fibre Channel, LUNs). CapEx hardware vs cloud OpEx.
- 3-2-1: 3 copies, 2 media types, 1 offsite. RAID = local redundancy. Backups = disaster recovery. You want both.