Use the SHR RAID calculator above to estimate usable capacity for Synology Hybrid RAID arrays. SHR is Synology's automated RAID system — it partitions drives into slices and spans them to maximize space, especially when mixing different drive sizes.
What Is Synology Hybrid RAID (SHR)?
SHR is an automated RAID management layer built on Linux mdadm and LVM. Released in 2010 with DSM 2.3, it was designed to solve the biggest frustration with traditional RAID: the inability to mix different-sized drives without massive capacity waste.
Traditional RAID 5 forces all drives to match the smallest disk in the array. If you have 2×2TB and 2×6TB drives, RAID 5 treats all four as 2TB drives — giving you only 6TB usable out of 16TB raw. SHR, by contrast, intelligently partitions each drive and creates multiple internal RAID arrays that are spanned together, delivering 10TB usable from the same hardware.
🧠 How SHR Slicing Works (4×2TB + 2×6TB Example)
All four 2TB drives and the first 2TB of each 6TB drive form a RAID 5 across 6 × 2TB slices → 10TB usable
The remaining 4TB "tails" on the two 6TB drives form a RAID 1 mirror → 4TB usable
Both RAID arrays are spanned via LVM into a single JBOD → 14TB total usable
Traditional RAID 5 on the same hardware: only 10TB usable (all limited to 2TB)
SHR delivers 40% more space on the exact same drives.
Understanding "Protection" & How Parity Works
If you've used Synology's RAID Calculator, you may have seen something confusing: a 4×8TB SHR array shows 21.8TB available but only 7.3TB of "protection." Does this mean only 7.3TB of your data is safe? No — all 21.8TB is protected. The "protection" number refers to the disk capacity used for redundancy, not the amount of data being protected. A single parity drive can protect any number of data drives.
RAID Is NOT a Backup
RAID protects against drive failure — not accidental deletion, ransomware, fire, or theft. Even SHR-2 with two-drive redundancy is not a substitute for proper backups. Always maintain a 3-2-1 backup strategy: 3 copies of your data, on 2 different media types, with 1 offsite. The Synology community on Reddit repeatedly stresses this: RAID = uptime and fault tolerance, backup = data survival.
How Parity Protects Your Data
Parity works like a simple math equation. Imagine storing the numbers 4 on drive A and 6 on drive B. The parity drive stores the sum: 4 + 6 = 10. If drive A fails, the system knows A + 6 = 10, so A must be 4. If drive B fails, 4 + B = 10, so B must be 6. One parity value can reconstruct any single missing number — it doesn't matter how many data drives there are, as long as only one is missing at a time.
In practice, parity uses XOR (exclusive-OR) — a binary operation that's faster and more compact than arithmetic — but the principle is identical. The system stores data1 XOR data2 XOR data3 = parity, and any single drive can be reconstructed from the remaining drives and the parity value. For SHR-2 (dual parity), a second parity calculation using a different formula allows reconstructing two missing drives simultaneously.
Why the Ratio Changes with More Drives
A common source of confusion: with 2×8TB SHR, the Synology calculator shows 7.3TB available and 7.3TB "protection" — a 50/50 split (it's a mirror, RAID 1). Add two more drives for 4×8TB SHR and you get 21.8TB available but still only 7.3TB "protection" — now a 75/25 split. The protection capacity stays at one drive's worth, but it now protects three times as much data. This is not a bug or a risk — it's how parity RAID is designed to work.
Drives × Size
Usable
Parity Space
Efficiency
How It Works
2 × 8TB SHR
7.3TB
7.3TB
50%
Mirror (RAID 1) — full duplicate
3 × 8TB SHR
14.6TB
7.3TB
67%
Parity protects 2 data drives
4 × 8TB SHR
21.8TB
7.3TB
75%
Parity protects 3 data drives
6 × 8TB SHR
36.4TB
7.3TB
83%
Parity protects 5 data drives
8 × 8TB SHR
50.9TB
7.3TB
87.5%
Parity protects 7 data drives
Parity space stays fixed at one drive's capacity regardless of total drives. More data drives = higher storage efficiency. SHR-2 uses two drives' worth (14.6TB for 8TB drives), protecting against two failures.
SHR vs RAID 5 — Capacity Comparison
Configuration
RAID 5 Usable
SHR Usable
SHR Advantage
4×4TB (identical)
12TB
12TB
Identical
4×2TB + 4×4TB
14TB
20TB
+43%
2×4TB + 2×8TB + 2×12TB
20TB
36TB
+80%
SHR-2: Dual-Drive Redundancy
SHR-2 is the dual-parity version of SHR, protecting against two simultaneous drive failures — equivalent to RAID 6 in fault tolerance. With identical drives, the capacity formula is simply (Number of Drives - 2) × Drive Size. With mixed drives, only the two largest drives are "lost" to redundancy, and the remaining space is maximized through intelligent partitioning.
SHR-2 is recommended for arrays of 6+ drives or when using drives larger than 8TB, where the probability of a second drive failure during a lengthy rebuild becomes significant. It provides the same protection as RAID 6 but without the rigidity of requiring all drives to match the smallest size.
When SHR Is the Right Choice
✅ Use SHR When
• You own a Synology NAS
• You plan to expand storage gradually
• You have (or may acquire) mixed drive sizes
• You want zero-configuration RAID management
• You value capacity efficiency over raw speed
❌ Skip SHR When
• You may move drives to a non-Synology NAS
• You need maximum write performance (use RAID 10)
• You're using TrueNAS/Unraid (use ZFS or unRAID pools)
• All drives are identical size (SHR = RAID 5/6)
SHR FAQ
What is SHR (Synology Hybrid RAID)?
SHR is Synology's automated RAID management system that allows mixing drives of different sizes while maximizing usable capacity. Unlike traditional RAID 5 which forces all drives to match the smallest disk, SHR partitions each drive into slices and creates multiple RAID arrays that are spanned together — only the single largest drive is effectively lost to redundancy.
How does SHR capacity compare to RAID 5 with the same drives?
With identical-size drives, SHR and RAID 5 produce the exact same usable capacity: (N-1) × drive size. But with mixed drives — say 4×2TB + 4×4TB — SHR delivers 20TB usable vs RAID 5's 14TB. That's a 43% capacity advantage.
Can I add drives to an existing SHR array?
Yes, this is one of SHR's standout features. You can add drives of any size at any time without rebuilding the entire array from scratch. The system automatically re-slices and spans the storage pool to incorporate new capacity. Traditional RAID typically requires a full array rebuild or identical drive sizes for expansion.
Can I move SHR drives to a different NAS?
SHR drives can be moved to another SHR-capable Synology NAS and the volume will be recognized automatically. However, SHR arrays cannot be read by non-Synology devices — the drives would need to be reformatted. This is a key limitation vs standard RAID or ZFS.
Why does Synology's calculator show different "protection" ratios for 2 vs 4 drives?
With 2 drives, SHR uses RAID 1 (mirroring) — the second drive is a full copy, so available = protection (50/50 split). With 3+ drives, SHR switches to parity (RAID 5 style), where one drive's capacity protects ALL data drives. The "protection" number stays at one drive's worth, but the usable space grows with each added drive — hence the ratio shifts from 50/50 toward 75/25, 87.5/12.5, etc. Your data is equally protected regardless of ratio.
Is RAID a substitute for backups?
No. RAID protects against drive failure so your system stays online — it does not protect against accidental deletion, ransomware, fire, theft, or controller failure. Even SHR-2 with dual-drive redundancy is not a backup. Always follow the 3-2-1 rule: 3 copies of your data, on 2 different media types, with 1 copy off-site.