Summary

This DfD is an informative technical paper meant to educate readers about the initial intentions around the U.3 interface standard, how it proceeded to fall short upon development, and why server users may want to continue using U.2 SSDs for their server storage needs.

Introduction

In our world of technology, we expect to see multiple generations of devices and standards, with each successive generation being faster and more feature-rich than the previous. We have seen this pattern so often that we expect version N+1 of anything to be better than version N in nearly all respects.

So, what about the new U.3 interface standard compared to U.2? Surprisingly, U.3 is not the next generation since it was not intended to replace U.2. It was originally conceived as a low-cost NVMe replacement for the SATA SSD. A lot has changed since the inception of U.3 and eventually the standard required U.3 SSDs to be backwards compatible to existing x4 U.2 SSDs. This requirement forced SSD vendors to either develop flash controller silicon with 6 PCIe lanes or to add mux chips to steer the existing PCIe lanes on the SSD. By doing so, U.3 SSDs have the following disadvantages:

• U.3 SSDs do not hit the same cost points as SATA SSDs
• U.3 SSDs do not have any cost advantage over U.2 SSDs (and may end up being more expensive)
• U.3 SSDs lost their ability to differentiate themselves from U.2 SSDs

U.3 has been touted as a way to enable a tri-mode backplane that will support SAS, SATA and NVMe drives to work across multiple use-cases. The tri-mode backplane claim was to reduce system costs, while providing an upgrade path so that users can later replace their existing SAS and SATA drives with higher performance NVMe SSDs. While a tri-mode backplane can technically support SAS, SATA, and NVMe drives, mixing SAS and SATA virtual disks behind a single controller is rarely done. Adding NVMe to the mix makes even less sense because NVMe SSDs are much higher performing than SAS or SATA drives.

Even an upgrade path from SAS or SATA drives to all NVMe SSDs is severely limited by the tri-mode controller. A high-performance controller has 16 lanes that can support, for example, 16 x1 devices. Replacing 16 x1 SATA SSDs with 16 x1 NVMe SSDs as originally envisioned by U.3 would make sense. However, because U.3 matched U.2 and with support for up to a x4 link, customers will likely not want to give up the higher performance the additional lanes provide. A 16-lane tri-mode controller could support only 4 x4 U.3 SSDs – not very many for such an expensive controller.

A SAS expander would normally be used to increase the number of SAS devices, but it cannot support PCIe as there are no tri-mode expanders. Additionally, a PCIe switch would normally be used to increase the number of NVMe devices, but it cannot support SAS or SATA devices. The result is that the system either suffers poor

U.3 performance or must incur the cost of additional tri-mode controllers. Thus, because U.3 combines the SAS and SATA lanes with the NVMe lanes, it is much more difficult and expensive to scale out the tri-mode solution to achieve high performance. The argument that U.3 allows system designers to develop a common set of backplanes that work across multiple use cases does not hold, as the difference in link widths and the inability to scale will push users to continue adopting solutions tailored to their specific needs.

 U.2 keeps the SAS and SATA lanes separate from the NVME lanes, allowing system designers to scale solutions independently with readily available SAS expanders and PCIe switches. Dell Technologies recognizes a wide range of customer requirements and provides solutions that are tailored to each market as opposed to a one-size-fits-all solution. To that end, Dell Technologies has developed high-performance, universal x4 drive bays that have been shipping on Dell PowerEdge servers for the last two generations. Dell Technologies also provides SAS and SATA-only solutions to reduce cost in entry-level systems. Next generation backplanes enable NVMe HWRAID which connects up to 8 NVMe SSDs at PCIe Gen4 x2. Direct connect solutions remain at PCIe Gen4 x4.

Conclusion

U.3 enables a tri-mode backplane which allows simple upgrades from SAS or SATA to NVMe, yet increases the base cost of a SAS or SATA solution. Moreover, unless the system is designed with sufficient lanes for the NVMe SSDs, the performance will be poor. The additional hardware required to obtain full NVMe performance negates any system cost benefits of the U.3 architecture.

Dell Technologies has demonstrated that the wide range of customer requirements can be met with SAS, SATA and U.2 drives, using designs targeted individually for performance or cost. Dell’s universal U.2 backplane takes advantage of the separation of SAS and SATA lanes from NVMe lanes to maximize NVMe performance, while maintaining SAS and SATA compatibility in a universal bay. This high-performance, universal U.2 backplane avoids the confusion and complexity brought by U.3. However, it is important to remember that the key is the backplane architecture, not the drive type. Dell’s backplane will work with U.3 SSDs, as well as U.2 SSDs, since U.3 SSDs are required to be compatible. Dell has also designed next generation backplanes to enable NVMe HWRAID which connects up to 8 NVMe SSDs at PCIe Gen4 x2. All of Dell’s direct-connect solutions remain at PCIe Gen4 x4.