U.2, U.3, and other server NVMe drive connector types (in mid 2022)
The other day I casually looked around to see how readily available U.2 NVMe drives were compared to SATA SSDs. In the process I saw some mention of '2.5" U.3' NVMe drives, which was a connector type I'd never heard of, and did some digging.
The short summary of U.2 is that it's NVMe drives in more or less the 2.5" SSD form factor (although according to Wikipedia, U.2 can also deliver two SATA lanes), with a different edge connector. Our recent experience with some U.2 based servers says that this works; our U.2 NVMe drives in drive carriers look and handle basically the same as SATA SSDs in drive carriers in other servers. To tell them apart, you have to either look at the back of the drive where the connectors are or notice the big 'NVMe' sticker on the front of the drive carrier.
U.3 is sort of an evolution of the U.2 connector and form factor, but it's sufficiently unloved that Wikipedia barely has a mention of it. The goal of U.3 is to create a 'tri-mode' standard where the same server drive bay can support U.3 NVMe, 2.5" SAS, or 2.5" SATA drives (and also the same server backplane and controller; see here, here, and here). A U.3 NVMe drive is backward compatible to U.2 drive bays, but a U.2 NVMe drive can't be used in a U.3 drive bay, presumably for reasons.
For people like us, ordinary 1U servers with U.3 drive bays would be reasonably attractive. We'd mostly use them with SATA SSDs, but if we had a server that could benefit from NVMe it would be easy to switch over to it. If we had an NVMe drive failure and had no spare for some reason, we could swap in a SATA SSD to get the server back on the air. And we wouldn't need specific spares for NVMe servers the way we do with U.2, because any server could be an NVMe server if we needed it to be.
(The natural number of 2.5" drive bays for a 1U server seems to be four, and with NVMe drives that only needs PCIe x16, which is pretty widely available.)
However if you do Internet searches for U.3 you'll soon discover that there's a competing set of standards for NVMe disks on servers, the EDSFF series, and some people feel that U.2 and U.3 are doomed in the face of them. The EDSFF form factors are specific to NVMe SSDs; there's no concession for backward compatibility to the 2.5" form factor.
I have no idea how this is going to shake out. People are still announcing new U.3 NVMe drives today, but there's EDSFF activity too. My biased perspective is that right now we're more interested in U.3's flexibility to choose between SATA and NVMe for SSDs than what EDSFF might deliver. But if EDSFF causes NVMe prices to drop to the level of SATA SSDs, sure, we'd be happy to go NVMe. It's flash storage either way, so if everything else is equal we'd rather have the faster version.