Synology SHR: How Hybrid RAID Calculates Usable Capacity
SHR (Synology Hybrid RAID) delivers the same redundancy as RAID 5 but uses a tiered algorithm to maximise usable space when drives are different sizes.
See exactly how much usable storage your configuration delivers.
What is SHR?
SHR is Synology's proprietary RAID implementation. With identical drives, it behaves exactly like RAID 5. One drive's worth of capacity is used for parity, and you get (n−1) drives of usable space. The difference only appears with mixed drive sizes.
How SHR Handles Mixed Drive Sizes
Standard RAID 5 requires all drives to contribute equal capacity. Limited to the smallest drive in the array. A 2TB, 3TB, 4TB, 4TB array in RAID 5 would use only 2TB from each drive, wasting much of the larger drives.
SHR splits the array into capacity tiers and creates separate RAID groups for each tier. This recovers the unused capacity from larger drives. The same 2TB+3TB+4TB+4TB array gives approximately 8.18 TiB usable under SHR vs 5.46 TiB under RAID 5.
The SHR Algorithm
SHR sorts drives by size and processes each size tier:
- Sort drives ascending: d1 ≤ d2 ≤ ... ≤ dn
- For each tier (unique size group), calculate the chunk = current size minus previous size
- Usable from tier = chunk × (number of drives in tier − 1)
- Sum all tiers for total usable capacity
Example: 2TB + 3TB + 4TB + 4TB drives in TiB: 1.82, 2.73, 3.64, 3.64
- Tier 1 (1.82 TiB chunk, 4 drives): 1.82 × 3 = 5.46 TiB
- Tier 2 (0.91 TiB chunk, 3 drives): 0.91 × 2 = 1.82 TiB
- Tier 3 (0.91 TiB chunk, 2 drives): 0.91 × 1 = 0.91 TiB
- Total before overhead: 8.18 TiB
The RAID calculator applies this algorithm automatically when you select SHR and enter mixed drive sizes.
When SHR Wins vs RAID 5
SHR and RAID 5 are identical with matching drives. Use SHR when:
- You are starting with mismatched drives (common when upgrading from a previous NAS)
- You plan to expand by replacing drives with different sizes over time
- You want maximum flexibility without planning a uniform drive upgrade path
Use RAID 5 instead of SHR when: you are running a non-Synology NAS (SHR is Synology-proprietary) or when you want to migrate to a non-Synology device in future (RAID 5 volumes can be read on any Linux system; SHR cannot).
Frequently Asked Questions
Can I convert an SHR array to RAID 5?
With identical drives, Synology allows migration from SHR to RAID 5 and vice versa via Storage Manager without data loss (the process rebuilds the array). With mixed drives, SHR cannot be converted to RAID 5 without data loss and reformatting.
Does SHR support hot spare drives?
Yes. One or two drives can be designated as hot spares in DSM's Storage Manager. Hot spares are excluded from the usable capacity calculation. The RAID calculator's hot spare field removes them from the drive count before calculating usable capacity.