Take a look at this teardown of a 2.5" Samsung 860 Pro:<p><a href="https://www.myfixguide.com/samsung-860-pro-ssd-teardown/" rel="nofollow">https://www.myfixguide.com/samsung-860-pro-ssd-teardown/</a><p>Notice how little of the case is actually occupied by circuitry: <a href="https://i.imgur.com/lzPTC3T.jpg" rel="nofollow">https://i.imgur.com/lzPTC3T.jpg</a><p>I think the better question than M.2 vs. U.2 is the "plugs directly into motherboard" form factor vs "uses a cable from the motherboard to plug into an enclosure for a separate circuit board that needs separate mounting points in the case" form factor.<p>Since forever, hard drives have come in 3.5" and 2.5" form factors, so when SSDs hit the market it made sense for them to come in backwards-compatible enclosures, in the same way it made sense for them to implement backwards-compatible protocols like SATA.<p>NVMe, as a protocol, breaks backwards compatibility, to great benefit, so I don't think it's surprising that breaking backwards compatibility on form factor happens at the same time.<p>U.2 seems ideal if you've already got a server chassis design that accepts 2.5" hot-swap SSDs, and you want to use the NVMe command set instead of SATA. For most consumer purposes, having M.2 storage that you just plug into the motherboard like you do RAM and the CPU is completely worth the switch away from the 2.5" drives.
U.2 is more popular in servers. M.2 is common because of laptops. It is used in servers mainly for boot drive or where someone is gaming the cost difference of enterprise drives. There are form factors like Intel's ruler if you really want density that bests even custom M.2 carriers for servers.<p>Another point the author may not be aware of, most server SSD sales happen as parts of bigger integration deals. The SKUs for many/most U.2 devices for instance might never hit something like Newegg or Amazon but might be moving in massive volumes at Supermicro or Sanmina or the like. This is in contrast to say 3.5" hard disks which spent most of their life forwarded by pushing consumer demand and the only difference in an enterprise disk may be some firmware knobs.
U.2, a.k.a. SFF-8639[1], in fact, is the successor of SAS.(It also allows legacy SAS and SATA usage.) So, it has a stronger enterprise inheritance. And so, the enterprise clients can even use the same disk cases/cages without any problem when updating.<p>Compared to M.2 form, U.2 has two benefits:<p>1. support hot-swapping(SAS-age enterprise character)<p>2. better thermal performance/heat dissipation(for larger shape)<p>for data centers, the future will be the EDSFF family[2](goodness of U.2+goodness of M.2). M.2 can still stay for the consumer market.<p>[1] <a href="https://en.wikipedia.org/wiki/U.2" rel="nofollow">https://en.wikipedia.org/wiki/U.2</a><p>[2] <a href="https://www.anandtech.com/show/13218/ssd-form-factors-proliferate-at-flash-memory-summit-2018" rel="nofollow">https://www.anandtech.com/show/13218/ssd-form-factors-prolif...</a>
"Most people prefer the simplicity of plugging a M.2 card into a motherboard connector rather than mounting a separate drive and running cables to it."<p>I love that part so much, I'm a complete convert. Considering replacing SATA SSDs with NVMe SSDs just to reduce the cable load inside my rig. Just wish PCIe cards with dual NVMe ports were more abundant.
It's interesting to hear that M.2 was the underdog in 2015. I recently built a new PC and made sure to get a motherboard with two M.2 slots precisely because of the "look ma, no cables" aspect of the connector. Even with SATA.. That's a big advantage in my book.<p>U.2 seems to be more popular in applications where drives may need to be changed out more frequently, like datacenters.
I have had the opportunity to work with an Epyc server with a full load of NVMe/U.2 direct connected to all those tasty PCIe lanes. Any given drive worked wonderfully, and writing simultaneously across many drives with DD showed very good performance. Unfortunately any attempt to use Linux native software RAID showed performance barely in excess of a single drive, even with a simple stripe. I don't blame the NVMe, but something in Linux RAID just doesn't scale in performance. I spent days tweaking it, ultimately it was a great disappointment.
Its unfortunate how U.2 does not receive much attention. How much M.2 devices can you probably fit into curent workstation/server motherboards? 3? U.2. takes place of SATA for being a dedicated connection to multiple storage devices and can fit so much more in same chassis.<p>If we need some sort of new-gen ssd raid, the unweildiness of m.2 mounting won't gonna cut it.<p>The u2 benefits, compared to m2 mentioned on wikipedia seems to miss the point - u.2 is meant to be important in server space where you can have 12 drives per 1U, all with frontal/top load. all these cards we have with m.2 are not meant and cannot be operated at storage-system scale.
If you want to run multiple U.2 drives, then OCuLink on the host side should interest you: both the quad port AOC-SLG3-4E4T and the OCuLink to U.2 cables are relatively cheap. <a href="https://forums.servethehome.com/index.php?threads/nvme-make-the-most-of-your-pci-e-slots-how-to-config-supermicro-boards-for-aoc-slg3-2e4t-et-al.17651/" rel="nofollow">https://forums.servethehome.com/index.php?threads/nvme-make-...</a><p>Another choice is a somewhat more usual four M.2 card and <a href="https://click.intel.com/u-2-to-m-2-ssd-cable-replacement-u-2-to-m-2-cable-for-pcie-nvme-supporting-intel-solid-state-drives.html" rel="nofollow">https://click.intel.com/u-2-to-m-2-ssd-cable-replacement-u-2...</a> this cable. To me this seems much more of a hack than OCuLink.
The answer is - as always - compatibility. There is nothing worse than a new interface. It limits the consumer base for a manufacturer, it limits the lifespan of a device, it limits the upgrade options for a customer, it raises the amount of permutations (and cost) required to target a common subset, etc, etc.<p>If there's an interface that solves the problems of the 80% that don't need more than one drive in their laptop or PC (let alone more than the two most laptops can fit if the manufacturers wanted) and it avoids all these issues... then the alternative isn't going anywhere.<p>It's the same story with SAS, despite its considerable superiority over SATA, it was never going to appear in consumer hardware.
Note the part about Intel limiting pcie lanes. It is a story that has been mostly ignored but it has shaped the pc industry in many ways, mainly to the detriment of Intel because it eliminates some of the seasons to buy a computer instead of a phone.
Nvme is an interesting development in storage.<p>It does away with layers of abstraction and gives you 4x pcie lanes pretty much straight to the device.