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PCIe SSD: Enhanced Speed

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PCIe definition: a PCIe SSD is a solid state drive (SSD) that connects to a computer system using a PCIe interface. PCIe stands for Peripheral Component Interconnect Express, which is also known as PCI Express, or PCI-E.

Modern computer systems are equipped with PCIe "slots" or sockets, and these sockets can be used to connect many different types of devices such as graphics cards and PCIe SSDs. Clearly, PCIe flash-based storage is a key element in today's data storage landscape.

PCIe

PCIe is a much newer flash storage interface standard, and this has gone through various increasingly fast iterations, including PCIe 2.0 and PCIe 3.0, which is the current PCIe spec used for SSD connectivity. (In fact the first NVMe SDD that supports PCIe 4.0 was released in July 2018, but PCIe 4.0 SSDs are not commonly available yet.)

There are various different PCIe slot types of differing lengths, because they may include varying numbers of "links" over which data can travel. The smallest PCIe slot contains one link and is known as a PCIe x1 slot. Other slots include PCIe X4, PCIe X8, and PCIe X16, which has 16 links and is currently the fastest PCIe interface commonly available.  

A PCIe 3.0 X16 interface offers a total bandwidth of 16 GBps, while PCIe 2.0 X16 offers 8 GBps. In contrast, a PCIe 1.1 X16 manages 4 GBps. All are vastly faster than the PCI interface's maximum bandwidth of 532 MBps.

Aside from the enhanced speed of PCIe compared to PCI, PCIe also offers other benefits, including advanced error detection and reporting. It enables the ability to hot-swap devices so that they can be inserted and detected without the need to reboot the system.

pcie

The Samsung 970 EVO is an example of a high performance PCIe.

PCI vs PCIe

PCIe should not be confused with plain PCI, which is a much older interface standard first proposed by Intel as far back as 1990, and was implemented widely in computer systems five years later. The internal architecture of PCIe resembles a local area network with each link being connected to a central switch in the computer. PCI differs in that all devices share the same parallel bus.

PCI slots are generally longer than PCIe slots, but the key difference is that the older PCI technology runs at a much slower speed than that which is attainable by PCIe SSDs. The standard 32-bit PCI slot has a maximum throughput of 133MBps, while a 64-bit PCI slot can run at up to 532 MBps.  

PCIe Technology

In practice, most SSD buyers will choose between a PCIe SSD, which uses a PCIe interface, or a SATA SSD, which uses a common modern alternative interface: the Serial ATA or SATA interface.

PCIe SSD vs. SATA SSD

PCIe SSD Benefits

The key benefit of PCIe SSDs is the huge performance boost that they provide compared to SATA drives, as described above.

In fact this performance gain is not the whole story, because as well as plain vanilla PCIe SSDs it is also possible to buy PCIe NVMe SSDs, which are faster still.

SATA SSD benefits

  • Compatibility: SATA is a much older interface for SSDs than PCIe, dating back to 2003. As a result, SATA interfaces are much more common, and that in turn means that SATA SSDs are likely to be compatible with many more older systems than PCIe SSDs.
  • Efficiency: SATA is also a less sophisticated interface than the newer PCIe interface, and that means that the energy consumption of SATA SSDs is likely to be lower than more modern PCIe SSDs. This may not be important for desktop users, but it can have important implications for battery  life for laptop users. It may also be important in large data centers where power and cooling costs can be considerable.
  • AHCI: SATA SSDs can be hot-swapped in and out of a system as long as the system is using AHCI (Advanced Host Controller Interface), which can usually be activated in the system's BIOS. AHCI also enables systems to use  a technique called native command queuing (NCQ), which can also enhance the performance of SATA SSDs. (PCIe SSDs can be hotswapped by default)  
  • Cost: SATA SSDs tend to cost significantly less than PCIe SSDs. This may be important for some purchasers, but this cost differential may not be a true comparison because SATA SSDs tend to offer poorer performance than PCIe SSDs. For example, A SATA 3.0 SSD may offer an effective data rate of about 560 MBps, while a PCIe 3.0 SSD may offer performance 3 to 6 time higher.

speed comparison

With each new generation, PCIe moves ever faster than SATA/SAS.

What is PCIe NVMe?

NVMe stands for Non Volatile Memory Express, a specification for accessing SSDs which are connected using a PCIe interface (in a similar way that AHCI is used with the SATA interface).

NVMe offers a performance boost by using the parallelism of the flash storage medium to reduce latency and increase performance. In fact NVMe is not restricted to flash storage: some of the very fast SSDs use NVMe to control Intel's 3D XPoint non-volatile storage medium which is an alternative to regular NAND (flash) memory.

PCIe types PCIe 2.0 vs 3.0

PCIe 2.0 dates back to 2007, offering a data transfer rate of 5 GTps (and throughput per lane of 500 MBps). That means that a 32 lane connector (i.e. PCIe X32) can support a throughput of up to 16 GBps.

PCIe 3.0 (sometimes called PCie gen3) was released three years later in 2010, and in theory it offers about double the performance of PCIe 2.0.  

PCIe SSD form factors

M2 PCIe SSD

One SSD form factor which is worth mentioning here is the M.2 PCIe SSD "gumstick" form factor mentioned earlier, which is popular in laptops and other systems where space is at a premium.

M.2 PCIe SSDs support PCIe 3.0 (as well as SATA 3.0 and USB 3.0) interfaces, and they can also benefit from the NVMe protocol, which offers significant performance benefits when running over PCIe.

U2 PCIe SSD

Another important form factor is the U.2 PCIe SSD form factor. U.2 is actually a connector, which uses up to four PCIe 3.0 lanes. U.2 PCie SSDs tend to be designed for the enterprise market, especially in solid state storage systems, and are often produced  as slim 2.5-inch devices, or thin cards similar to M.2 PCIe SSDs

PCIe SSD Speed

PCIe 3.0

So what kind of performance can be expected from today's PCIe SSDs? The fastest PCIe SSD is impressive. The most common benchmarking software to come up with accurate figures are CrystalMark and AS SSD.

Some of the fastest PCIe SSDs currently available use Intel's 3D XPoint storage media, such as the Intel Optane SSD 900P series. This uses a PCI3 3.0 X4 interface and NVMe, offering:

  • Sequential read speed: up to 2,500 MBps
  • Sequential write speed: up to 2,000 MBps
  • Random read speed: 550,000 IOPS
  • Random write speed: 500,000 IOPS

More typical PCIe 3.0 X4 performance figures are:

  • Sequential read speed: up to 2,300 – 3,300 MBps
  • Sequential write speed: up to 1,300 - 2,000 MBps
  • Random read speed: 140,000 – 400,000 IOPS
  • Random write speed: 150,000 - 350,000 IOPS

PCIe 2.0

Most modern PCIe SSDs are designed for PCIe 3.0 slots, but since PCIe is backwards compatible they can still be used in older systems with PCIe 2.0 slots. The resulting performance will be about half the performance if the SSD were connected to a PCIe 3.0 slot, so typical PCIe 2.0 performance figures would be:

  • Sequential read speed: about 1500 MBps
  • Sequential write speed: about 1400 MBps
  • Random read speed: about 200,000 IOPS
  • Random write speed: about 180,000 IOPS

speed comparison

The speed of PCI Express made a huge leap from 2.0 to 3.0, increasing its speed advantage or Serial ATA.

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