Synology NAS Storage: SHR, DSM Overhead, and Usable Capacity
Synology DSM reserves approximately 10 GB per installed drive for system partitions and swap. On top of that, SHR, Btrfs filesystem metadata, and optional snapshot reserves further reduce what is available for your files.
See exactly how much usable storage your configuration delivers.
How Synology DSM Manages Storage
Synology DiskStation Manager (DSM) creates storage pools and volumes on your installed drives. Before any user data is stored, DSM allocates approximately 10 GB per installed drive for system data, swap space, and internal partitioning. This figure is published in Synology's official RAID calculator documentation.
On a 4-drive NAS, that is 40 GB reserved before a single file is written. On an 8-drive NAS, 80 GB. The deduction is per drive, not per pool.
Filesystems: Btrfs vs ext4
DSM defaults to Btrfs for most volume types. Btrfs provides snapshots, checksumming, and deduplication at the cost of approximately 4% metadata overhead. ext4 is also available and uses only ~2% overhead, but lacks snapshot support.
The filesystem choice affects usable capacity and can be selected when creating a volume in DSM's Storage Manager. You cannot convert between filesystems without reformatting.
RAID Options on Synology
- SHR (Synology Hybrid RAID). Single-drive redundancy, optimised for mixed drive sizes. Equivalent to RAID 5 with identical drives.
- SHR-2. Two-drive redundancy. Equivalent to RAID 6 with identical drives. Requires 3+ drives.
- RAID 5. (n−1) × drive capacity. Three drives minimum.
- RAID 6. (n−2) × drive capacity. Four drives minimum.
- RAID 10. Half total capacity, strong read/write performance. Four drives minimum (even number).
- RAID 1. Mirrored pair. Two drives, single drive capacity usable.
- JBOD / RAID 0. No redundancy. Full capacity, no fault tolerance.
Typical Synology Configurations
Estimates below use default calculator settings: 10 GB/drive system overhead, 4% Btrfs metadata, 0% snapshot reserve, 10% headroom buffer.
| Config | Advertised | Est. Usable (TiB) | For Files (TiB) |
|---|---|---|---|
| 2 × 4TB, SHR | 8 TB | 3.22 | 2.90 |
| 4 × 4TB, SHR | 16 TB | 10.44 | 9.39 |
| 4 × 8TB, RAID 5 | 32 TB | 21.0 | 18.9 |
| 6 × 4TB, RAID 5 | 24 TB | 16.8 | 15.1 |
| 4 × 4TB, SHR-2 | 16 TB | 6.77 | 6.09 |
To run calculations for your specific drive count and size, use the NAS RAID Calculator. If you want to estimate how long a pool rebuild would take after a drive failure, see the RAID Rebuild Time Estimator.
Frequently Asked Questions
How much space does Synology reserve for the system?
Synology DSM reserves approximately 10 GB per installed drive. This is a vendor-published figure from Synology's official RAID calculator footnotes. On a 4-drive NAS that is 40 GB total.
Should I use SHR or RAID 5 on Synology?
With identical drives, SHR and RAID 5 deliver identical usable capacity. SHR's advantage is with mixed drive sizes. It uses multiple RAID tiers to maximise usable space across different capacities. If you plan to expand by adding a larger drive later, SHR handles this more efficiently than RAID 5.
Does Synology support adding drives to expand a RAID pool?
Yes. SHR and RAID 5/6 pools support online expansion by adding drives or replacing drives with larger ones. The pool rebuilds in the background without taking the NAS offline. SHR handles mixed sizes during expansion more gracefully than standard RAID 5.