Side-by-side specifications of the Synology DS1621+ and DS1821+, from official datasheets. Differences are highlighted.
| DS1621+ | DS1821+ | |
| CPU | AMD Ryzen V1500B | AMD Ryzen V1500B |
| Cores / threads | 4 | 4 |
| RAM (base) | 4 GB | 4 GB |
| RAM (max) | 32 GB | 32 GB |
| Drive bays | 6 | 8 |
| Expandable to | 16 | 18 |
| M.2 slots | 2 | 2 |
| Network | 4x 1GbE RJ-45 | 4x1GbE RJ-45 |
| 10GbE | optional | optional |
| Max raw capacity | 256 TB | 108 TB |
| Hot-swap | yes | yes |
| Form factor | desktop | desktop |
| Released | 2020 | 2020 |
The DS1821+ is the stronger pick if you want more headroom: both use the same Ryzen V1500B CPU, but the DS1821+ has 8 bays vs 6 on the DS1621+, supports up to 32GB RAM like the DS1621+, and gives you two extra drive bays for capacity and RAID flexibility. Both support 10GbE expansion and M.2 NVMe SSD cache via add-on cards, so the real decision is mainly 6-bay cheaper/smaller vs 8-bay more scalable.