Can I use RAM as Cache for a secondary storage device

Solution 1:

DRAM is used as cache by default on most operating systems. Research how the caching tiers and file system performance work for your OS of choice. Watch disk performance metrics before and after installing more memory.

Think about where the bottlenecks should be in theory. If the RAID 5 array is on magnetic spindles, maximum sequential is maybe 200 MB/s per drive and probably is the limit. If those are fast solid state, the 10 Gb Ethernet could be the limit. For the latter case, consider 25 Gb links, maybe multiple of them.

Perhaps you have heard the joke, RAID 0 refers to the 0 data left after one drive failure, a complete loss. I am skeptical anyone using RAID 0 is aware of the risk. Destroy one disk as a test, and determine if recovery via backups or rebuild is acceptable.