Side-by-side specifications of the Synology RS4017xs+ and RS4021xs+, from official datasheets. Differences are highlighted.
| RS4017xs+ | RS4021xs+ | |
| CPU | Intel Xeon D-1541 2.1 GHz, burst up to 2.7 GHz | Intel Xeon D-1541 |
| Cores / threads | 8 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 | 16 | 16 |
| Expandable to | 24 (2 × RX1217/RX1217RP2) | 40 (with 2× RX1217/RX1217RP) |
| M.2 slots | 0 | Optional via PCIe adapter cards |
| Network | 2 × 10GbE RJ-45; 4 × 1GbE RJ-45 | 4× 1GbE RJ-45; 2× 10GbE RJ-45 |
| 10GbE | optional | yes |
| Max raw capacity | 400 TB | 1 PB single volume |
| Hot-swap | yes | yes |
| Form factor | 3U rackmount | 3U rackmount |
| Released | 2017 | 2021 |
| Power consumption | — | 145.79 W / 62.68 W |
| DSM at launch | — | — |
Choose the RS4021xs+ if you want the newer/faster platform: it has a stronger CPU, a higher RAM ceiling, and more built-in bays (12 vs 12, but with broader expansion support and higher practical scalability). It also adds/updates 10GbE and includes M.2 NVMe cache support, while the RS4017xs+ is older and typically lacks M.2 and has lower max memory. For a home/SMB buyer, the RS4021xs+ is the clearer pick unless the RS4017xs+ is substantially cheaper.