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All you need to know about Solid State Drives: Interfaces, protocols & form factor

Get to learn what makes SSDs tick.

You see it, all your friends see it …. and they are having a hell of a time making fun of you and your old laptop. You just can’t take it any more. Its time to shop for a new laptop. You make up your mind on all the specs you need to have in your new machine then hit the market… Amazon, Best Buy, WalMart. After a few days of searching, you’ve narrowed down to three of the best laptops within our budget. They have similar specs and an almost similar price. They all have solid state drives (SSDs) and you can’t wait to blow the roof with those crazy speeds you hear about… but only if you can settle on one. Somehow, you just can’t get your head around those SSDs. Which of these machines has the best one. Hmmm…

Or perhaps your computer had a hard disk drive when you bought it, but you read somewhere in the manual that it has an extra slot that can handle a solid state drive. You need that upgrade so bad, but just don’t know how to go about it. And you just don’t have the extra money nor the time to look for someone to do it for you.

While these situations may not apply to everyone, they are real to lots of folks.

All you need to know about Solid State Drives

In order to get a clear picture of SSDs, there are a few things you need to have a basic understanding about.

In this article, you will:

  1. Learn about the two main physcical interfaces
  2. Find out about the different SSD form factors
  3. Find out about the different types connectors and which ones are most common

We’ve broken this article into four section. In these sections, we shall discuss about interfaces, form factors, connectors and communication protocols. Once you are done with these sections, you’ll be ready to hit the market.

Let’s brush through some definitions and some basic understanding of connectors, form factors and protocols.

  • Connectors

A connector is a device that physically links or connects two things together. In our case, we’re talking about the different types of physical connectors between solid state drives (SSDs) and the motherboard.

  • Form factor

“Form factor” refers to the size and physical configuration of a device.

Image of the different SSD form factors
  • Protocols

A protocol (or communication protocol) is a set of rules by which computers communicate with each other.

Watch this video by ThieJoe to give you a better understanding of this topic

INTERFACES

The two main interfaces are SATA an PCIe. The third but less commonly known interface is SAS (Serial Attached SCSI).

A basic understanding of interface here means the physical method of the connection between the SSD and the the motherboard.

On a higher level, interface refers to the hardware transportation layer including voltage, current, and physical pin definition,

An SSD connects to the host using an “interface,” which adheres to a specific “protocol’, where protocol refers to the set of rules, standards, command sets and drivers between the storage and the operating system.

The choice of interface will determine the amount of data that can be transmitted within a period of time (bandwidth), the delay before data transfer actually begins after the transfer instruction is sent (latency) and the capability to expand the system or network to adapt to growing workloads (scalability). It will also determine whether your SSD will have hot-swap or hot-plug capabilities. All of these are important considerations in enterprise usage.

  • SATA

SATA is currently the prevalent interface for connecting an SSD to the host. It affords the convenience of being used interchangeably with SATA-based HDDs. Transfer rates for first-generation SATA began at 1.5 Gb/s, and the latest generation, SATA III, provides a native transfer rate of 6 Gb/s. SATA uses the Advanced Host Controller Interface (AHCI) command protocol, which was designed for slower mechanical drives with spinning disks.

  • mSATA (max throughput 600 MB/s)
  • SATA (max throughput for SATA 3 is 600 MB/s)
  • Some M.2 models (max throughput of 600 MB/s)

 

M.2 Interface. Note that an SSD with an M.2 interface has two notches. One with a PCIE interface has one notch.

Further, single SATA slots in the SATA Express connector also use SATA bus.

  • PCI-Express

Though PCIe was originally designed as an interface for linking motherboard-mounted peripherals like graphics or wireless network cards, its excellent features for scalability, minimal latency, high bandwidth, and high performance makes it an emerging interface favorite, enabling SSDs to blaze past the SATA III limitation of 6 Gb/s transfers.

The interface specification optimized for NAND flash and next-generation solid-state technologies is known as NVM Express or NVMe. NVMe delivers twice the performance of SAS 12 Gb/s and four times the performance of SATA 6 Gb/s.

  • SATA Express interface (as a whole connector it utilizes PCI Express x2 – max throughput 2 GB/s)
  • U.2 interface (utilizes PCI Express x4 – max throughput 4 GB/s)
  • Some M.2 models (up to PCI Express x4 – max throughput 4 GB/s)
  • PCIE interface (it can go up to PCI Express x16, which means max throughput of ~16 GB/s)
PCIe Interface (back). Note that an SSD with a PCIe interface has one notch and one with a M.2 interface has two notches
  • Serial Attached SCSI (SAS)

SAS is designed for high-performance enterprise requirements and is seldom used by the larger majority.

 

As you can see, there are M.2 SATA drives and M.2 PCI Express drives. It is essential to distinguish them because it determines in which scenarios the drive will and will not work, and is also an indicator of performance. The M.2 SATA drive won’t be faster than 600 MB/s.

It is good to know that some motherboards run an M.2 slot with only PCI Express x2 speed, which could bottleneck the fastest drives. Also, some older motherboards only support M.2 SATA drives and PCI Express drives won’t work on them at all.

FORM FACTORS

Initially, SSDs were seen as HDD replacements and came in HDD sizes such as 1.8″, 2.5″, 3.5″ and 5.25″. In late 2010, SSDs consisting of flash chips and controllers attached to a circuit board were used in ultra-thin mobile computers. These SSDs were long, thin and narrow, and had no enclosure.

Choosing the right form factor is important, as it defines whether it fits in the system chassis, how many can fit, and if it can be replaced without powering off the system (hot-plug/hot-swap functionality).

  • Traditional HDD Size

SSDs with enclosure come in 1.8″ for removable ultra-mobile applications, 2.5″ and 3.5″ for desktop and enterprise systems, and the less common 5.25″ for special-purpose appliances like backup devices.

  • Solid state modules (M.2)

Flash memory chips reside in a dual in-line memory module (DIMM) or similar form. They may use a standard HDD interface like SATA.

The M.2 standard provides higher performance and capacity while minimizing module footprint. M.2 modules connect either via SATA or PCIe, come in multiple widths and lengths, are available in soldered down or connectorized type, and can have single- or double-sided components. All soldered down modules are single-sided and are intended to be used in low-profile applications. (Note: M.2 modules are neither hot-swappable nor hot-pluggable.)

M.2 NVMe

M.2 SATA 2242, 2260 and 2280
  • mSATA

Just roughly the size of a business card, an mSATA SSD adopts the PCI Express Mini form factor and connector. Though it can easily be mistaken for a PCIe mini card, an mSATA SSD signaling conforms to the mSATA standard, which is based on the SATA storage interface supporting maximum data transfer rates up to 6 Gb/s. It plugs into an existing PCIe Mini card slot but will function properly only if connected to a SATA host controller. Compatible slots will be labeled as either dedicated for mSATA or shared with PCIe mini. A PCIe Mini card installed on a dedicated mSATA slot will not function properly.

  • Flash memory cards

Memory cards also use flash memory and are used either as removable or embedded storage. ATP’s industrial flash memory cards are built for rigorous computing conditions in automotive applications and industrial environments.

  • Other form factors

These include embedded USB (eUSB) and USB Drives.

CONNECTORS

SSDs link/ interface with computers through several connector types. These include SATA, mSATA, SATA Express, PCIe, M.2, U.2. Others SSDs are even soldered directly onto the motherboard. However, the most common consumer option are SATA and M.2. PCIe is slowly catching on. Lets have a look at some of them here.

  • SATA connector

SATA or S-ATA or Serial ATA are all abbreviation for Serial Advanced Technology Attachment, which is an evolution of the Parallel ATA (PATA) interface that was developed for connecting a host system to peripheral devices, such as hard disk drives and optical storage drives.

This is the most common type of connector. It’s the cable that connects to your hard disk drive (HDD), and you’re probably familiar with it. There are 3 generation of SATA, their main difference between them being the bandwidth they support. These are:

SATA I1.5 Gb/s
SATA II3 Gb/s
SATA III6 Gb/s

 

It is important to note that Gb/s is Gigabit per second, not Gigabyte per second.

 

Male SATA Connectors

Female SATA connectors
  • mSATA connector

Unlike the SATA connector, this standard does not make use of cables. The SSD is inserted directly into the slot.

mSATA interface

mSATA connector

The mSATA looks very similar to the M.2 although the two are incompatible. This standard is no longer in use, except perhaps in older machines. Applications include netbooks and other devices that require a smaller solid-state drive.

  • M.2 (NGEF) connector

The name M.2 as with mSATA refers to both the drive and the connector. This is perhaps the most widely used connector today after SATA.

Similar to mSATA, the drive is inserted directly into the slot and is held in place by a single screw. The drive itself comes in a variety of lengths (otherwise referred to as form factors). The first two digits indicate the width (mm) while the last two the length (mm)

  • 2230
  • 2242
  • 2260
  • 2280
  • 22110
Several sizes of M.2 drives can be fit into this adapter

 

The most common is 2280 and 2242, while the least common is 22110 because of its long length. Some notebooks can only support a maximum length of 60 or 42 millimeters, so be careful to note this before you make your purchase.

The most common consumer form factor is 2280, which means it is 22 mm wide and 80 mm long. As a size, 22110 is somewhat rare, so not all motherboards support it. It is good to know that most notebooks can take only take SSDs with a maximum length of 42 mm or 60 mm. Check your pc before you move go shopping lest you end up with a drive that’s too long to fit your machine.

  • SATA Express connector

Image of SATA Express cable and connector. Image credits: legitreviews.com

The SATA Express connector is for the most part dead. It was aimed at providing a faster connection between 2.5” SSDs over PCIe but failed because of its two-lane limitation. Thus is was abandoned, leaving only limited accessories, like USB-C panels, to utilize it.

It saw its dawn with a vision to eliminate the bottlenecks that came with the SATA interface. That was made possible by blending PCIe and SATA standards to create a faster interface with the capacity to delivered up to 16Gbps up from the 6Gbps with the SATA connector. However, it did not meet the destiny that it was built for.

  • U.2 (SFF-8639) Connector

Formerly SFF-8639, its was renamed U.2 to make it more marketable to the general public. However, it ended up finding better finding more demand in the enterprise market rather than the general consumer market.

Basically, the U.2 connector is a pumped-up SATA Express connector. Its advantage over the SATA Express connector is that it is more compact in size and offers higher transfer speeds.

A U.2 (SFF-8639) connector cable

This is a 2.5” NVMe SSD. The wide connector plugs into the drive, while the smaller one plugs into the motherboard

The connector for disks is totally different than the connector for motherboards, and only a handful of disks with this interface exist. The smaller plug goes to the motherboard while the wider one is for the drive.

Unlike normal SATA and SATA Express cables, the U.2 cable consists of several small, shielded cables that increase its cost – a further discouragement common everyday computer users.

  • PCI-Express connector

PCIe or PCI-E is an abbreviation for Peripheral Component Interconnect Express.

It is a standard type of connection for internal devices in a computer. PCI Express is largely used to refer to both the expansion slots on the motherboard as well as the type of expansion cards that go into it.

While computers may have a blend of various types of expansion slots, PCIe is considered the main internal interface. As a matter of fact, more and more motherboards than not today are being manufactured with the PCI Express slots only.

The main advantage of the PCIe interface is that it makes it possible to have very high-bandwidth communication between the motherboard and the device connected to it.

The massive steps made by PCIe connectors have made it desirable as the connector of choice for several hardware. These include videocards, RAM, sound cards, CPUs, hard drives and solid state drives. There exists PCIe cards for all this hardware. So don’t be surprised to see graphics card or sound card with a PCIe connector.

Image of the different PCIe slots (white in color). The smallest is (x1), the middle is (x4) while the others are (x16) but can accomodate (x8) harware. The larger the connector, the more data lanes it has and the more bandwidth between the card and the host.

PCIe Versions: 4.0, 3.0, 2.0 or 1.0

You might be wondering, what do the numbers after PCIe mean on an SSD, a motherboard or even a graphics card. For instance, you can find this written on an SSD: 1TB PCI-Express 3.0 2U X8 SSD.

The number 3.0 refers to the latest version of the PCI Express specification that is supported by that hardware, be it a motherboard or SSD. The X8 on the other hand refers to the slot size is can fit in. We’ll talk more about slot size later.

1TB PCI-Express 3.0 2U X8 SSD

 

There are currently four versions of PCI-Express available, with PCI-Express 5.0 set for release in 2019.

Bandwidth (per lane)Bandwidth (per lane in an x16 slot)
PCI-Express 1.02 Gbit/s (250 MB/s)32 Gbit/s (4000 MB/s)
PCI-Express 2.04 Gbit/s (500 MB/s)64 Gbit/s (8000 MB/s)
PCI-Express 3.07.877 Gbit/s (984.625 MB/s)126.032 Gbit/s (15754 MB/s)
PCI-Express 4.015.752 Gbit/s (1969 MB/s)252.032 Gbit/s (31504 MB/s)

Isn’t it amazing how far we’ve come? From a bandwidth of 6Gb/s with SATA to a massive 252Gb/s per lane with PCI-Express 4.0 (Equivalent to 4032.512Gb/s for the 16 lanes).

PCIe Sizes

There are different types of PCIe connector sizes. Often times will see the connectors or hardware with inscriptions such as PCI Express x1, PCI Express x4, PCIe Express x8 or PCI Express x16. When that happens, take a deep breath and thank the heavens because you know that it just refers to size.

The number after the x indicates the physical size of the PCIe card or slot, with x16 being the largest and x1 being the smallest. The larger the slot, the more data lanes between that connector and the motherboard, and thus the higher the bandwidth it can support. So larger is better here. However, there is usually a cost increase incurred with higher lane counts.

  • ‘PCIe x1’ connections have one data lane
  • ‘PCIe x4’ connections have four data lanes
  • ‘PCIe x8’ connections have eight data lanes
  • ‘PCIe x16’ connections have sixteen data lanes
  • ‘PCIe x32’ connections have thirty-two data lanes (currently, these are VERY rare)
PCI-Express x1 SSD. Note that the notch only comes after pin 11

The notch in any PCIe card comes after pin 11. That means that regardless of whether its an x1 or x16 card, the notch is at the same position.

Because of this, an x16 PCIe slot can accommodate an x16, x8, x4 and x1 card. Similarly, an x8 slot can accommodate an x8, x4 or x1 card. In short, a shorter PCIe card works fine in a longer slot.

A slot can accommodate a large card than itself as long as it is open. However, it will operate at slower speeds than it would have ordinarily been able to.

PROTOCOLS

There are two main protocols used in computing today. These are AHCI and NVMe.

Video by Techquickie

  • AHCI

AHCI stands for Advanced Host Controller Interface, which is an the older protocol, optimized for standard magnetic drives. Usually you could switch the AHCI disk to use even older IDE (Integrated Drive Electronics) protocol, but that is better avoided. This video by Techquickie does a good job at explaining this.

  • NVMe

NVMe on the other hand stands for Non-Volatile Memory Express. This is the new protocol designed for fast SSD drives. If the drive utilizes this standard, it is likely a performance model. NVMe protocol can be used only over PCI-Express bus-based drives. If the M.2 drive uses a SATA interface, it can’t be an NVMe model.

NVMe Form factors

NVMe drives come in three basic form factors, the Add-in card (AiC), the 2.5” (U.2) and the M.2.

All-in-card NVMe form factor – An Intel NVMe PCIe SSD

 

2.5″ NVMe form factor – A 2.5″ SSD connected to PC through a U.2 Connector

Image from bit-tech

M.2 NVME form factor – An M.2 NVMe SSD

 

Edwin M

Professionally a Mechanical Engineer and a Tech Nerd at heart, Edwin is above all a passionate hobby guitar player and proud gospel music listener. He writes reviews of gadgets he would love to buy and recommend.