Comment by dgacmu

To dive slightly into that: You don't necessarily want to sacrifice space for a read cache disk: having more space can reduce writes as you do less replacement.

But where you want endurance is for a ZIL SLOG (the write cache, effectively). Optane was great for this because of really high endurance and very low latency persistent writes, but, ... Farewell, dear optane, we barely knew you.

The 400GB optane card had an endurance of 73 PB written. Pretty impressive, though at almost $3/GB it was really expensive.

This would likely work but as a sibling commenter noted, you're probably better off with a purpose-built, high endurance drive. Since it's a write cache, just replace it a little early.