M.2 is a solid-state drive (SSD) form factor primarily used as an operating system boot solution however depending on the endurance of the M.2 drives, they may also be suitable for read-intensive or mixed workloads.
Lenovo ThinkSystem servers support M.2 drives in one of three ways, depending on the server selected:
One or two drives, mounted in an M.2 adapter, vertically in a dedicated slot
Two or four drives, mounted horizontally in an M.2 adapter