Side-by-side specifications of the Synology RS3618xs and RS4021xs+, from official datasheets. Differences are highlighted.
| RS3618xs | RS4021xs+ | |
| CPU | Intel Xeon D-1521 | Intel Xeon D-1541 |
| Cores / threads | 4 cores / 8 threads | 8 cores / 16 threads |
| RAM (base) | 8 GB DDR4 ECC UDIMM | 16 GB DDR4 ECC UDIMM |
| RAM (max) | 64 GB | 64 GB |
| Drive bays | 12 | 16 |
| Expandable to | 36 total with 2 expansion units | 40 (with 2× RX1217/RX1217RP) |
| M.2 slots | 0 | Optional via PCIe adapter cards |
| Network | 4 × 1GbE RJ-45 | 4× 1GbE RJ-45; 2× 10GbE RJ-45 |
| 10GbE | optional | yes |
| Max raw capacity | 216 TB | 1 PB single volume |
| Hot-swap | yes | yes |
| Form factor | 2U rackmount | 3U rackmount |
| Released | 2018 | 2021 |
| Power consumption | 142.76 W / 55.05 W | 145.79 W / 62.68 W |
| DSM at launch | — | — |