It's not easy to understand exactly what you want. There seem to be two possibilities:
- You want a version of Redis that holds its active database in RAM and also writes/reads some of the database to SSD/NVME (flash) memory.
- You want to run Redis on a server that will swap some active RAM pages to SSD/NVME (flash) memory.
Since you're looking at Redis documentation, implying you expect Redis to have explicit support/control of the data stored in flash, it's likely you're looking for #1.
Have a look at this discussion thread
from several years ago, talking directly about #1 and indirectly about #2. The creator of Redis gives the reason Flash is not good for Redis in the middle of the thread, but for full details see the timing comparison in the last message of the thread
(disclaimer: written by me).
Summary: Compared to RAM, Flash is way too slow. Putting the speed comparison into human timescales, if reading from RAM takes one second, reading from flash takes five and a half hours. (best-case HD takes four and a half days)