SSD Cache layer doesn't make sense given upcoming NVMe


Ok last post on the file server topic. Vlad pointed out that there are two storage layers here. That is near line SAS plus a PC3500 when it's available for high speed ssd.
So with 20 slots in a Norco 4220 you could easily add a caching Sata ssd layer as well. So you end up with 32-256GB DDR4, 2-4GBps NVMe SSD, 800MBps Sata ssd and then the main 140MBps hard disk layer.
So so what is a good caching ssd that zfs can use?
There are lots of consumer SSD reviews but not many enterprise SSD roundup but  Anandtech shows the Micron 500DC  Is a good example. About $600 for 500MB. Seems expensive but they are 40% over-provisioned.
Tom's IT Pro has a good overview including SAS drives and the pricing is such that I wonder if we should really recommend a SSD cache layer between the Intel PC3500 PCIExpress system and hard drives. The pricing is about the same as the NVMe layer and performance is about 4x less.

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