QNAP NAS Storage: System Reserve, Snapshot Space, and Usable Capacity
QNAP QTS applies two significant overhead deductions that surprise many buyers: a ~70 GB system reserve per storage pool, and a default 20% snapshot reserve. Together these can consume 25-30% of your usable capacity before any files are stored.
See exactly how much usable storage your configuration delivers.
QNAP QTS Storage Architecture
QNAP QTS organises storage into storage pools (the physical RAID set) and volumes (logical partitions within a pool). Each layer carries its own overhead.
- Storage pool overhead: ~70 GB per pool for system data (vendor-published)
- Static volume overhead: ~20 GB per static volume
- Snapshot reserve (default): 20% of storage pool capacity
The snapshot reserve is the largest single deduction and is often overlooked. On a 4-bay NAS with 4 × 4TB drives in RAID 5, the default 20% reserve removes approximately 2.1 TiB from what you can actually use for files.
How to Reduce the Snapshot Reserve
In QTS Storage Manager: go to Storage & Snapshots → Storage Pool → right-click the pool → Manage → Snapshot Space Management. You can reduce the reserve to 0% if you do not use snapshots, or set a fixed GB allocation rather than a percentage.
RAID Options on QNAP
- RAID 5. Most common for 4+ bay QNAP. (n−1) drives usable.
- RAID 6. Two-drive fault tolerance. (n−2) drives usable.
- RAID 1. Mirror. Single drive capacity usable.
- RAID 10. Half total capacity. Strong performance.
- JBOD / RAID 0. No redundancy. Full capacity.
QNAP does not support SHR, TRAID, or ZFS RAID types. It uses standard Linux mdraid under the hood.
Typical QNAP Configurations
Estimates use defaults: 70 GB pool overhead, 20% snapshot reserve, 2% ext4 metadata, 10% headroom.
| Config | Advertised | Est. Usable (TiB) | For Files (TiB) |
|---|---|---|---|
| 4 × 4TB, RAID 5 | 16 TB | 8.51 | 7.66 |
| 4 × 8TB, RAID 5 | 32 TB | 17.3 | 15.6 |
| 4 × 4TB, RAID 6 | 16 TB | 5.49 | 4.94 |
| 8 × 4TB, RAID 6 | 32 TB | 17.5 | 15.7 |
Frequently Asked Questions
Why does QNAP show so much less space than advertised?
The 20% default snapshot reserve is the main reason. It reserves one-fifth of your storage pool before you use a single byte. Combined with the 70 GB pool overhead and binary conversion, you can lose 35-40% of advertised capacity on a modest configuration. Reducing the snapshot reserve to 0% recovers the most space.
Does QNAP support ZFS?
QTS 5.1+ introduced QuTS hero mode with ZFS support for select models. Standard QTS uses ext4. If your model runs QuTS hero, the ZFS overhead table applies (approximately 3.1% metadata, plus TrueNAS-level complexity). The RAID calculator's QNAP mode uses ext4 defaults.