Currently, all Synology models with M.2 NVMe slots can only use the M.2 SSD for caching. However, in DSM 7.2, M.2 volume creation is supported, albeit with some caveats
1. You must have DSM 7.2 to support this feature
2. You must have Synology M.2 SSDs
3. The only series that support this are DS923+, DS723+, DS1522+, DS1621+, DS1821+, and DS1621XS+
An M.2 NVMe volume would be very fast. But M.2 NVMe is also notorious for overheating, so good ventilation and heatsinks are usually recommended for M.2 NVMe. Synology has tested its M.2 SSDs for thermal performance and that is probably why they have forced volume creation with Synology drives only
Aside from overheating, M.2 NVMe SSDs are mostly not hotswappable - Synology has not confirmed that yet. M.2 NVMe capacities are also limited. But despite all that, there will be users who want a faster drive and can take advantage of this new feature from Synology
