Untangling terms: M.2, NVMe, USB-C, SAS, PCIe, U.2, OCuLink

I have been working in data recovery industry for 10 years now. And in this post, I would like to explain a few abbreviations that sometimes get people puzzled. If you want to learn how these terms are used and how they correlate, this post is for you.

Look at the table with short descriptions of the most vague terms, I am providing more details further down in this text.

M.2

M.2 is a physical interface and a popular form-factor for solid state drives (SSD). It was initially designed for compact devices.

Its most popular size is 2280 (which stands for 22 mm wide by 80 mm long).

Example: Samsung 850 EVO

Read /Write speed: 520 MB/s

There are 2 physical interface keys. You can meet one or both of them used on a M.2 SSD connector. Those are B and M keys which define the gap between pins.

M.2 SATA and PCIe

While M.2 is a form-factor, SATA and PCIe are two different communication interfaces used for M.2. They define protocols (languages) and speeds.

  • M.2 SATA SSD reads/writes below 600 MB/s (SATA 3.0 specification defines the actual speed)
  • M.2 PCIe 3.0 SSD can read/write as fast as 4000 MB/s (defined by PCIe 3.0 spec)
  • M.2 PCIe 4.0 SSD can read/write as fast as 8000 MB/s (defined by PCIe 4.0 spec)

Speeds as mentioned above are just theoretical thresholds. It is always important to pay attention to the tech specs of each particular SSD.

Interesting fact

M.2 SSD cannot support both SATA and PCIe interfaces simultaneously. Though, some motherboards have hybrid slots supporting drives with both interfaces.

NVMe

This host controller and specification, from a certain point of time, has been developing with intentional SSD support. NVMe became the high-performance alternative to SATA/AHCI standard. Unlike M.2, it says nothing about the form-factor.

NVMe solid state drives use PCIe slot, which provides advantage of parallel PCIe lanes to maximize data transfer speed.

NVMe drives are becoming more and more trendy these days. Samsung, the leader in SSD market, switched their mass production to NVMe.

Example: NVMe Samsung 960 PRO

Read speed: 3500 MB/s, Write speed: 2100 MB/s

Interesting fact

NVMe protocol enables to run up to 65535 commands simultaneously. It is impossible with SATA or SCSI protocols.

SAS

SAS stands for Serial Attached SCSI. SAS is a physical interface designed for server use, whereas SCSI is a communication protocol.

SAS hard disk drives are often compared to SATA HDDs. In this context, I will say that SAS drives are usually faster than SATA drives. SAS drives show 10000/15000 RPM (revolutions per minute) versus 5400/7200 RPM of SATA drives. What’s more, SAS drives are more reliable and robust, as they are made of better components than similar SATA devices.

There are cons, though. SAS drives tend to be loud, so you don’t want to use them at home. SAS drive temperature is also higher, which may cause issues. And of course, SAS is more expensive than SATA.

To sum up, SAS drives are normally used in servers.

Example: SAS Seagate with 15000 RPM

Read/Write speed: 300 MB/s

Interesting fact

You can visually differentiate SAS device by joined power and data connectors. There is a gap in SATA interface connector, while there is none in SAS connector.

U.2

U.2 is a physical interface designed mainly for SSD NVMe drives that used in servers.

Visually, it resembles SAS connector. Technically, U.2 is SFF-8639 standard while SAS is SFF-8482 standard.

U.2 interface looks like SAS

Being designed for server use, U.2 supports hot swap whereas M.2 does not. As for speed, up to 4 PCIe lanes enable to reach 4 GB/s performance. In theory. The practical example of quite popular U.2 SSD model is below.

Example: Intel 750 SSD U.2

Read speed: 2100 MB/s, Write speed: 800 MB/s

Interesting fact

There are adapters for connection of U.2 drives to M.2 ports.

USB-C

USB-C (which is short for USB Type-C) is a new hardware interface, which can provide:

  • a reversible connector
  • support of many communication protocols (Thunderbolt 3, HDMI, Display Port)
  • power delivery at 20V (100W)

There is a rule of thumb due to the variety of protocols: one should always check what’s behind a specific USB-C port in a device.

These days external drives with USB-C interface grow more popular.

Example: WD My Passport

Read/Write speed: 430 MB/s

USB 3.1

USB 3.1 is not the same as USB Type-C. USB 3.1 (Gen 2) is a standard increasing performance to up to 1250 MB/s. USB 3.0 allows half the throughput.

Interesting fact

USB-C with USB 3.1 Gen 2 support demands appropriate cable type.

OCuLink

OCuLink is a physical interface that stands for “Optical Copper (Cu) Link” and represents a cable version of PCI Express. OCuLink 2 (the last version) enables incredible performance up to 16 GT/s (8 GB/s total for ×4 PCIe lanes).

OCuLink port

Another great feature is flexibility given OCuLink motherboard port can be used in different modes. For instance, you can connect an NVMe drive via OCuLink cable. Then change corresponding motherboard’s jumper, and the port becomes available for connection of 4 SATA drives via another OCuLink cable.

OCuLink to U.2 (NVMe) cable
OCuLink to 4 SATA cable

Interesting fact

Initially, they expected to bring OCuLink into laptops for connection of powerful external GPU boxes. It turned out to be a rare use. Instead, OCuLink became popular for PCIe interconnections in servers.

That’s all for now, guys! I hope I shed some light on drive-related questions arising once in a while. Please feel free to ask a question or comment below.

Sincerely,
Vitaliy Mokosiy

CTO in Atola Technology. Gamification enthusiast. Agile proponent