What is a PCIe SSD?

Do you need one?

PCIe OCZ Revodrive SSD

SSDs have revolutionized computer storage, but with changes coming so rapidly, it hasn't been easy to keep up with the state of things. New generations of drives pop up just about every year, and terms like PCIe SSD, M.2, and NVMe get kicked around a lot at the high end. So, what does it all mean? How do you buy the best drive for your computer.

The Advantages of PCIe SSDs Over SATA Drives

Interfaces on a computer motherboard operate at different speeds. Just like you'd expect the internal components of a computer to communicate faster than something connected over USB, there are different bandwidth limits to internal interfaces, too. SATA has been the main interface used for connecting hard drives to a motherboard for several years. It works well, and with traditional hard drives can't max out its transfer capabilities, but SSD technology can. SATA relies on internal wires running from a drive to ports on a motherboard. It's not all that direct, but it gets the job done and allows for flexibility in your drive placement.

Traditional Hard Drive

PCIe has its history elsewhere. It's the high speed interface used for components like graphics cards that require a massive amount of data bandwidth at extremely high speeds. PCIe devices plug directly into the motherboard and pass data much more directly to the CPU at a higher rate. With PCIe, SSDs are more limited by their ability to read and write than their ability to transfer data.

How Much Faster Is a PCIe SSD?

PCIe is clearly faster, but how much faster is it? There are some easy estimates that can help spell that out.

The current iteration of SATA is SATA III. It supports a theoretical top speed of 6Gb/s, which works out to about 600MB/s of data transfer. It's certainly a respectable speed, but PCIe's capabilities will shift your perspective quite a bit.

PCIe is a little more complex to break down. First, there are PCIe 1.0, 2.0, and 3.0 sockets. 3.0 is the newest, but you'll still find 2.0 slots on some motherboards. Assuming the board you're using is a PCIe 3.0 board, there are still lanes to consider. PCIe connections are split into lanes. There are usually 4, 8, and 16 lane sockets, and you can identify them by size on the board. The large 16 lane ones are where you'd typically plug in a graphics card.

Solid State Drive

PCIe 3.0 has a theoretical speed of 1GB/s per lane, meaning a PCIe 3.0 x16 socket has a theoretical cap of 16GB/s. That's an insane amount of speed for a hard drive. Now, a usual PCIe SSD will more likely use 4 or 8 lanes, but the potential is still worlds better than SATA.

Those numbers really are theoretical, though, and not what your practical performance will look like. If you take a look at real SSDs, the speeds they advertise are much more grounded, but the benefit is still clear. The Samsung 860 EVO claims a max sequential read speed of 550MB/s and a max sequential write speed of 520MB/s. The closest comparable PCIe drive, the Samsung 960 EVO, has a reported 3.2GB/s max sequential read speed and 1.7GB/s max sequential write speed; it only uses 4 PCIe lanes. It isn't hard to see the benefit.

NVMe and M.2

There are a couple more terms that get kicked around along with PCIe drives: NVMe, and M.2. They are, thankfully, not complicated.

Samsung 970 EVO NVMe M.2 Drive

M.2 refers to a PCIe form factor designed specifically for SSDs. M.2 is more compact than standard PCIe, and only accepts M.2 form factor devices, which are pretty exclusively hard drives. M.2 was designed to provide an interface to allow SSDs to use a PCIe interface without interfering with, or taking slots from, more traditional PCIe devices like graphics cards. M.2 is also very common in laptops because it usually lays flat into the motherboard, taking very little space.

NVMe stands for Non-Volatile Memory Express. Non-Volatile memory is any kind of storage memory. Volatile memory refers to something like RAM that's constantly overwritten and doesn't remain after reboots. NVMe is a protocol designed specifically for PCIe hard drives to allow them to communicate more quickly. The goal of NVMe is to get SSDs to behave more like RAM because RAM uses very similar technology and moves much faster than SSDs.

Hopefully that untangled the current state of solid state drives. Things will continue evolving, but expect the use of PCIe hard drives, especially in the form of M.2 drives, to keep expanding in popularity. If you're considering a new SSD for your computer, PCIe is definitely worth a thought.