Synology SHR-2: Dual Redundancy with Mixed Drive Sizes
SHR-2 extends Synology Hybrid RAID to dual-drive redundancy. It tolerates two simultaneous drive failures while still maximising usable space across mixed-size drives.
See exactly how much usable storage your configuration delivers.
SHR-2 vs SHR-1 vs RAID 6
SHR-2 is to RAID 6 what SHR-1 is to RAID 5. With identical drives, SHR-2 and RAID 6 produce the same usable capacity. With mixed drives, SHR-2 uses the tiered algorithm to recover capacity that RAID 6 would waste.
- SHR-2 requires a minimum of 4 drives (3 drives is the minimum to have usable data with 2 parity)
- Capacity formula: (n−2) drives usable at the smallest size tier, with tiered recovery above that
- Protects against: any two simultaneous drive failures
When to Use SHR-2
SHR-2 is appropriate when:
- You have 4+ drive bays and are storing irreplaceable data
- You want to survive a double-drive failure during a RAID rebuild (rebuilds put stress on remaining drives)
- You have or plan to have mixed drive sizes and want maximum capacity efficiency
With identical drives on a 4-bay NAS, SHR-2 gives 2 drives of usable space (same as RAID 6). The usable TiB will be (n−2) × real_drive_capacity − system overhead − filesystem metadata.
Frequently Asked Questions
How much less space does SHR-2 give compared to SHR-1?
SHR-2 uses one additional drive's worth of capacity for the second parity layer. On a 4-bay NAS with identical drives, SHR-1 uses 3 drives of usable space and SHR-2 uses 2 drives. On a 6-bay NAS, SHR-1 gives 5 drives and SHR-2 gives 4 drives of usable space.
Can I migrate from SHR-1 to SHR-2?
Synology DSM supports online migration from SHR-1 to SHR-2 without data loss, provided you have enough drives. The migration rebuilds parity in the background and the NAS remains accessible throughout.