U.2 to PCIe Adapter for NVME SSDs
If you plan on using NVME SSD as a cache device and you don’t have a NVME slot in the ESXi host. You could either buy a NVME SSD in PCIe form factor like the Intel P4600 2TB or Samsung PM1725 1.6TB. Both these SSDs are highly recommended, but they are also more expensive than other NVME SSDs on the market. And there is only a limited selection for NVME SSDs in PCIe form factor.
Another option is to use the Startech U.2 to PCIe adapter that lets you connect a U.2 NVME SSD to a spare PCIe slot in the ESXi host. Though the latencies are a little higher because of this additional U.2<> PCIe adapter, the overall latencies are still very low compared to other components in the storage IO path. The one advantage to using this adapter is that you can now choose a lower capacity or cheaper NVME SSD, since there is a larger number of NVME SSDs in U.2 form factor than NVME SSDs in PCIe form factor.