Synology NAS Storage: SHR, DSM Overhead, and Usable Capacity
Synology DSM reserves approximately 10 GB per installed drive for system partitions and swap. On top of that, SHR, Btrfs filesystem metadata, and optional snapshot reserves further reduce what is available for your files.
See exactly how much usable storage your configuration delivers.
How Synology DSM Manages Storage
Synology DiskStation Manager (DSM) creates storage pools and volumes on your installed drives. Synology's RAID Calculator counts about 10 GB per installed drive for the DSM system and swap partitions, taken from the pool.
On a 4-drive NAS, that is 40 GB before a single file is written. On an 8-drive NAS, 80 GB. The deduction is per drive, not per pool.
Filesystems: Btrfs vs ext4
When a volume is created, Synology reserves 4% of its capacity for Btrfs metadata, or 2% for ext4. DSM offers both when you create a volume in Storage Manager.
RAID Options on Synology
- SHR (Synology Hybrid RAID). One-drive redundancy, built for mixed drive sizes. With identical drives it gives the same capacity as RAID 5.
- SHR-2. Two-drive redundancy. Synology requires at least four drives for an SHR-2 pool.
- RAID 5. (n − 1) × the smallest drive.
- RAID 6. (n − 2) × the smallest drive.
- RAID 10. (n ÷ 2) × the smallest drive, an even number of drives.
- RAID 1. A mirror: every drive holds a copy, so the capacity is one drive however many you add.
- RAID 0 / JBOD. No redundancy. Synology's calculator counts RAID 0 as the sum of every drive.
Classic RAID sizes every member to the smallest drive in the pool; the space above it is unused. SHR recovers that space with mixed sizes.
Worked Layout Examples
Layout capacity with identical drives, before DSM's reserves:
- 2 × 4 TB, SHR: 4 TB (one drive's worth, mirrored)
- 4 × 4 TB, SHR or RAID 5: 12 TB
- 4 × 8 TB, SHR or RAID 5: 24 TB
- 6 × 4 TB, SHR or RAID 5: 20 TB
- 4 × 4 TB, SHR-2 or RAID 6: 8 TB
DSM then takes about 10 GB per drive and 4% (Btrfs) or 2% (ext4) of the volume. Your computer may show the result in TiB, which is the same space as a smaller number.
To run your own drives, including mixed sizes and the full breakdown, use the RAID calculator. To estimate how long a rebuild would take after a drive failure, see the RAID Rebuild Time Estimator.
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
How much space does Synology reserve for the system?
About 10 GB per installed drive for the DSM system and swap partitions, as Synology's RAID Calculator counts it, plus 4% of the volume for Btrfs metadata or 2% for ext4.
Should I use SHR or RAID 5 on Synology?
With identical drives, SHR and RAID 5 give the same usable capacity. SHR's advantage is mixed drive sizes: classic RAID counts every drive at the smallest, while SHR uses the extra space in tiers. If you expect to grow with larger drives later, SHR can use them sooner.
Can a Synology pool grow by adding or replacing drives?
Synology allows adding drives to JBOD, RAID 5, RAID 6, RAID F1, SHR and SHR-2 pools (not RAID 0, RAID 1 or RAID 10), and replaces drives one at a time. On an SHR pool of equal drives, capacity grows only after at least two drives (SHR) or four drives (SHR-2) have been replaced with larger ones. The calculator's expansion planner shows each step.