Frequently Asked Question
Synology SHR vs RAID: fault tolerance, mixed drives and expansion
QUICK ANSWER
Synology Hybrid RAID (SHR) manages a storage pool using compatible drives and can make better use of mixed capacities than a conventional RAID layout. A single-drive SHR pool has no drive-failure protection. SHR-1 needs at least two drives for one-drive fault tolerance; SHR-2 needs at least four drives for two-drive fault tolerance.
Applies to: Synology NAS models that support SHR. The model, drive compatibility, DSM version and current pool layout determine available expansion and conversion options.
Understand what SHR protects
| Layout | Drive-failure tolerance |
|---|---|
| One-drive SHR | None. Failure of that drive can make the data unavailable. |
| SHR-1 with sufficient drives | One drive, provided the pool is healthy before the failure. |
| SHR-2 with sufficient drives | Two drives, provided the pool is healthy before the failures. |
SHR is optional and is not available on every Synology model. Supported alternatives can include Basic, JBOD and conventional RAID levels; check the options for your exact model. RAID does not protect against deletion, ransomware, theft or every rebuild failure. Keep a separate, tested backup.
Why mixed drive sizes can give different capacity
A conventional RAID5 layout generally uses the same capacity from each member, based on the smallest drive. SHR can form additional redundant sections from larger drives where the combination permits it. Extra capacity on one larger drive alone may remain unusable until another suitable drive is present.
Illustrative legacy example: 500 GB, 1 TB, 1.5 TB, 2 TB and 2 TB drives in a conventional one-drive-redundant layout. Figures exclude filesystem and system overhead.
The same illustrative drives arranged as SHR. This is an explanation, not a capacity promise for every supported combination.
Expand or change protection safely
- Check the pool and backup. In Storage Manager, record the RAID type, member-drive sizes, pool health, volume capacity and any active repair. Resolve degraded storage before planning an ordinary expansion.
- Add a drive only when DSM offers it. Use an empty supported bay and a compatible drive meeting the pool’s capacity requirements. A spare drive smaller than the smallest existing member is not a general expansion option. Check the wizard’s actual eligibility before purchasing.
- Replace drives one at a time. Follow DSM’s supported replacement process and let the pool finish repairing before replacing another member. Usable capacity increases only after the layout has enough suitable larger drives; replacing just one does not guarantee an immediate increase.
- Distinguish expansion from conversion. Adding a second equal-size drive to a one-drive SHR pool adds fault tolerance, not extra usable capacity. On supported layouts, Change RAID Type can convert SHR-1 to SHR-2; this has been supported since DSM 6.1 when all requirements are met. Do not assume every pool or bay count qualifies.
- Allow time and capacity. Repairs and conversions can take a long time and affect performance. Keep stable power and free space, and use Storage Manager’s reported state rather than a fixed completion-time promise.
Confirm the result
Storage Manager reports the intended RAID type and a healthy pool. Check whether any additional pool capacity must also be allocated to the volume, then verify files and the independent backup.
Before going further: Do not initialise an existing member, remove several drives together or interrupt a repair to speed it up. If a second drive reports errors during recovery, stop making changes and seek case-specific advice.
If you need help
Include NAS model, DSM version, RAID type, pool health, bay count, every member size and the proposed change. Use the Synology support checklist if the wizard does not offer the expected option.
Updated 261108