fnOS system drive, cache & RAID
The OS layer is where most first-time fnOS builds get slow or fragile. Get the system partition, file system and pool layout right and the rest is easy — the drive picks themselves are on our fnOS drive picks page.
System partition sizing
- Dedicated small SSD — the cleanest option. fnOS\'s OS + app store + metadata all live here; it reads constantly and rebuilds in seconds if it dies.
- Minimum ~64 GB — the practical floor for the system partition. Smaller works but leaves little headroom for app images and logs.
- Never on your data pool alone — if the system partition shares a drive with a degraded pool, you lose both at once. Keep them separate or on a mirror.
Do you actually need an SSD cache?
Skip it for a home file server. Add one only if you have a small, fast, sustained-write workload in front of slow spinning media — a 4K streaming box or a large-download server. The bigger wins at the same money are more RAM and a proper CMR drive, not a cache card. Confirm any part number in the SMR/CMR checker.
File system: ext4 vs Btrfs vs ZFS
| File system | When to choose it | Cost |
|---|---|---|
| Btrfs | The pragmatic default — snapshots + self-healing RAID, little fuss. | Low config, moderate RAM. |
| ZFS | Maximum robustness & scrubbing, heavy users, big pools. | RAM: ~1 GB per TB of vdev. |
| ext4 | Lightest, fastest, simplest — spend budget on drives. | Lowest; no built-in snapshots/RAID. |
RAID layout for a healthy pool
- 2 drives → RAID 1 (mirror). Simplest; half capacity.
- 3 drives → RAID 5 (one-parity) for file shares, or two-drive RAID 1 + a hot spare for media.
- 4+ drives → RAID 6 (two-parity) with CMR/enterprise disks — the standard for big home pools.
- Always pair with 3-2-1 — RAID survives a drive, not ransomware. Keep one offline or off-site copy: 3-2-1 for NAS.