Even though NVMe SSD support for cache in storage arrays on "prosumer" grade NAS is a big step, we still have a long way to go. Ideally, these systems will all include high-throughput interfaces like 10-gigabit Ethernet (and beyond) and NVMe acceleration support. The Synology DS1819+ compromises in that aspect, but that's not the only area where it falls short.
The add-in card doesn't support M.2 22110 form factor cards, the standard for enterprise M.2 SSDs. With 22110 support, we could have used larger Optane SSDs, or even Samsung 983 DCT drives with hardware power-failure protection. The extra length allows Samsung, and other SSD manufacturers to include power capacitors that purge the data in the DRAM to the flash in the event of a sudden power outage event.
In the future, we should see more support for the M.2 22110 form factor, but as of now, it's a rare feature found only in datacenter and enterprise servers. The drives cost slightly more than consumer models (but available through the channel and Amazon), but designed specifically for these workloads.
To maximize compatibility, we plan to use the Intel Optane SSD 800P drives in our prosumer NAS reviews that support NVMe acceleration. The M.2 2280 form factor will maximize our compatibility until we see the market support the larger form factor. The 800P also reduces the size of data held in the cache and that allows us to look at overall system performance and how the systems work with cache a little better than using very large drives.
Our choice doesn't mean the Optane 800P is the best solution for your business' workload. You can use our results today as a guide to face the right direction. In my own server, I recently replaced an LSI PCI Express SSD with an Intel Optane 900P 480GB drive that works in concert with a Mellanox 40GbE card to distribute data across our overpowered home network with roughly 20 devices randomly pulling data. In my personal testing, we used IMG Burn to read movie data and write back to the NAS a Blu-Ray Ultra (4K) ISO file. The Optane 900P cache configuration allowed the conversion to happen nearly twice as fast as the old LSI cache.
As they say, your mileage may vary. That said, it's difficult to beat 3D XPoint memory technology for low queue depth performance. Intel doesn't specifically list NAS acceleration for these products, but in our testing, the performance benefits are hard to match dollar for dollar.