Interfaces (M.2, U.2) and protocol (NVMe)
The successor of SATA600 was supposed to be the SATA-Express standard, a flexible connector that combines two physical SATA600-connectors and makes it possible to send PCI-Express signals through this connector. If the connector is not used as SATA-Express, two SATA-drives can be connected. The downside was that the bandwidth was 'only' four times higher than SATA600, which seemed much at first glance, but paled in comparison with the M.2-standard that was developed for laptops - this standard has a maximum speed of eight times SATA600. At the moment SATA-Express is dead: not a single drive was released for it, and the connectors are slowly but surely disappearing.
M.2 seems to be the standard for connecting modern storage, although this compact connector was originally meant for use in laptops and mini-PCs. M.2 is only a form factor, but a flexible one: the underlying interface can be SATA600 as well as PCI-Express 2.0 or 3.0 with two or four lanes. In the beginning only a few motherboards had an M.2-slot, and this was nearly always with a maximum of four PCI-Express 2.0 lanes. Because of this, they could only offer half of the maximum M.2 bandwidth.
Since the enormous expansion of the amount of PCI-Express lanes of the chipset with Skylake, the situation is reversed. Now even most motherboards in the budget-segment have an M.2-slot. The four extra lanes of the Kaby Lake chipsets also add to this: most mid-range motherboards have two or more M.2-slots. The new AMD Zen processors and chipsets also support M.2. The highest positioned Zen-processor and chipset can provide two M.2-slots with maximum bandwidth.
Slowly but surely another standard is also rising, aside from M.2. This standard offers the same amount of bandwidth, but works in a slightly different way: U.2. This standard is a lot like old SATA-drives: it is a connector and the drive has to be mounted in the case. With Skylake, some high-end motherboards had a U.2-connector, but now it is more common, as you can read in our review of Z270-motherboards. The SSDs available for U.2 are rather limited: in this test we have a total of zero, although the Intel 750 SSD is also available in a U.2-form factor.
U.2 has its own advantages and disadvantages. It is more difficult to fit in small laptops and Mini-ITX systems, but drives with this form can deal more easily with heat than an M.2 drive placed directly on the board. A 'box' is also less fragile than what is basically a bare PCB. Considering the heat issues that some M.2 SSDs have, we would not be surprised if this leads to an increase in popularity of U.2. Nevertheless the question remains if this standard will see a breakthrough or not. U.2 is supported by Intel, but other manufacturers are not exactly following suit. Still, Samsung stated that their PCI-Express SSDs will also be released with a U.2-interface in the future - provided that the standard becomes more popular. This is the well-known chicken and egg problem.
One of the advantages of PCI-Express SSDs is that nearly all of them support a new protocol, called NVMe. The important difference between its predecessor AHCI is that NVMe was designed with SSDs in mind, which means that the protocol is better geared towards the strong points of solid state storage. AHCI only allowed 32 commands to be sent at once to the drive, now this is increased to four billion - more than enough, even for SSDs. The CPU-overhead is significantly reduced as well. In a previous test we saw that the same drive (SM951 256GB) with NVMe was significantly faster than with AHCI, so this definitely has a practical effect.