Storage tiering
ZFS Tiers
| Tier | Device | Purpose |
|---|---|---|
| Hot | ARC (RAM) | Frequently accessed objects |
| Warm | L2ARC (SSD) | Recently accessed, overflows RAM |
| Small files | Special vdev (NVMe/SSD) | Metadata + files < special_small_blocks |
| Cold | HDD pool | Everything else |
ZFS manages promotion and eviction transparently. Reads of cold objects are promoted into ARC automatically. Small files land on the special vdev at write time.
Configuration
# Enable small file offload to special vdev (files up to 128K)
zfs set special_small_blocks=128K tank/putfs
See ZFS tuning for special vdev setup, ARC sizing, L2ARC, and SLOG configuration.
Without ZFS
| Tool | Purpose |
|---|---|
| OS page cache | Automatic hot tier (RAM) |
| bcache / dm-cache | SSD caching for HDD volumes |
| LVM cache | Tiered logical volumes |