4

I notice a lot of U.2 adapters on sale, seem to show what looks like a U.2 to SFF-8643 cable, or SFF-8643 PCIe connector/adapter, instead of the expected U.2 to SFF-8639 cable or SFF-8639 PCIe connector/adapter. (Example 1 ; Example 2)

  1. Are SFF-8643 and SAFF-8639 cables, connectors and adapters electrically compatible? Or are SFF-8643 deployed as 2 incompatible variants ( for use with SAS and U.2) that have the same physical form factor but are electrically incompatible?
  2. If compatible, does this mean I can plug any U.2 2.5" SSD into any SFF-8643 baseboard/PCIe adapter (not just into SFF-8639 connectors/adapters), provided the cable/adapter has physically compatible terminations at both ends? Would HBAs such as LSI-93xx with SFF-8643 connectors be compatible? Alternatively, what criteria apply for compatibility, how to check?
  3. SFF-8643 with SAS is typically used with (eg) 4 way fanout cables. What are the implications for SFF-8643 fanout if used with U.2 drives? Is fanout possible with U.2 drives (albeit at reduced individual bandwidth)? Or are fanout cables only made for SFF-8643 to SAS/SATA, not SFF-8643 to U.2?

I've looked but can't find a definitive answer, either on Google, or on this site, or on superuser.com. There are similar questions but none specifically clarify these points

Stilez
  • 714
  • 7
  • 15

1 Answers1

2

1. Electrical Compatibility

"Are SFF-8643 and SAFF-8639(U.2) cables, connectors and > adapters electrically compatible?"

Yes, they are electrically compatible but U.2 comes with an External Power Supply, since SFF-8643 does not have 12v, 5v, 3.3v pins.

2. How to check?

"does this mean I can plug any U.2 2.5" SSD into any SFF-8643 > baseboard/PCIe adapter?"

Yes - the most important thing you should check is when SFF-8643 port soldered within a PCIe Adapter or Motherboard have onboard SAS Controllers. Some of the adapters you might find online have parallel connections to PCIe, if you are planning to install SATA or SAS only drives on SFF-8639 or U.2, it will not work. SAS Controllers will make SATA or SAS Drives work.

3. Fanout Cables - Bandwidth, and Support for U.2, SAS, and SATA

"SFF-8643 with SAS is typically used with (eg) 4 way fanout cables. What are the implications for SFF-8643 fanout if used with U.2 drives?"

  • a single SFF-8643 is designed to support both 4x SAS Lanes and 4x PCIe Lanes.
  • SAS Controllers are more likely bottlenecked if all SAS Lanes are active.
  • PCIe is directly connected to either the CPU or a PCIe Controller Extender - PCIe have better low latency than SAS Controllers.

Or are fanout cables only made for SFF-8643 to SAS/SATA, not SFF-8643 to U.2?

Yes Mostly, Fanout Cables comes with SFF-8643 to SATA & SFF-8643 to SAS (it's important to note that some PCIe Cards can have no onboard SATA/SAS controllers). However, SFF-8643 to U.2 fanout cables? - It is technically possible to split x4 Lanes of PCIe to x1 for U.2 but most NVMe drives use x4 Lanes of PCIe. You can still use NVMe on x1 PCIe slot but you will loose trice the speed.

MasterC0de
  • 21
  • 4