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 own RAID layout. With identical drives it gives the same capacity as RAID 5: one drive's worth goes to parity and you get (n − 1) drives of usable space. The difference only appears with mixed drive sizes.
How SHR Handles Mixed Drive Sizes
Classic RAID 5 sizes every drive to the smallest one in the pool. A 2 TB, 3 TB, 4 TB and 4 TB set in RAID 5 uses only 2 TB of each drive.
SHR splits the drives into capacity tiers and protects each tier separately, which recovers most of the space above the smallest drive. The same 2 + 3 + 4 + 4 TB set gives 9 TB of layout under SHR against 6 TB under RAID 5.
The SHR Tier Method
- Sort the drives by size.
- Each step up in size is a tier: the chunk is that size minus the size below it.
- A tier is usable when at least two drives reach it; it gives chunk × (drives reaching it − 1).
- Add the tiers together.
Example: 2 TB + 3 TB + 4 TB + 4 TB
- Tier 1 (2 TB chunk, 4 drives): 2 × 3 = 6 TB
- Tier 2 (1 TB chunk, 3 drives): 1 × 2 = 2 TB
- Tier 3 (1 TB chunk, 2 drives): 1 × 1 = 1 TB
- Total layout: 9 TB, before DSM takes about 10 GB per drive and 4% (Btrfs) or 2% (ext4) of the volume
The RAID calculator applies this automatically when you choose SHR and enter mixed sizes, and shows any space left unused.
When SHR Wins Over RAID 5
SHR and RAID 5 are identical with matching drives. SHR helps when:
- You start with mismatched drives (common when moving drives from an older NAS)
- You plan to grow by replacing drives with larger ones over time
On an SHR pool of mixed sizes, Synology says to replace the smallest drives with drives at least as large as the largest already in the pool. On equal drives, capacity grows only after at least two drives have been replaced.
RAID keeps a NAS running through a drive failure. It is not a backup: deletion, ransomware, a failed rebuild or a dead NAS can still take everything, so keep a separate copy.
Frequently Asked Questions
Does SHR support a hot spare?
Yes. A hot spare for an SHR pool must be at least as large as the largest drive in the pool. Spares are not counted in usable capacity; the calculator's hot spare field takes them out of the layout.
Can I change an SHR pool to RAID 5 later?
Check Synology's Storage Manager documentation for your DSM version before planning a change of RAID type; with mixed drives, RAID 5 would count every drive at the smallest.