Understanding SSD form factors and interfaces

Understanding SSD form factors and interfaces

SSD’s have been transformative to the entire storage market. The use of solid state memory instead of spinning platters changed how thin, light, rugged, and power efficient modern laptops are.

However, understanding them well enough to upgrade your system has not been easy. Turns out, there have been not only a bunch of different form factors, but interfaces as well. Most guides I found do a terrible job explaining the differences – and often use form factor and interface types interchangeably in confusing ways.

Lets start our journey in understanding different SSD form factors and interfaces with termenology:

Form factor: The form factor is the physical dimensions for the drive. The form factors tells you if the drive will physically FIT in the system. They tell you nothing about whether the drive will actually WORK with your system – or even plug in.

It is possible to find two drives that have the same form factor, but be based on very different physical or interface technologies. For example: it’s possible to find a 2.5″ hard drive that uses traditional physical platters or solid state memory. It’s also possible to find 2.5″ drives that use the SATA interface or IDE (or many others too).

Interface: The interface of a drive tells you how the drive communicates and transmits data with the rest of the system. The interface is often (but not always) revealed by the type of power/data plugs that the drive has.

It is possible to buy a drive with the right interface for your system, but find out it won’t physically fit in the system. It is also important to know that, just like USB ports, some interfaces have the same physical plugs, but support many different speeds of data transfer. It’s possible to buy a drive with the right interface, but find that it’s nowhere near as fast as the interfaces allows (such as plugging in a USB 1.x device to a USB 3.x port).

Traditional hard drive form factors

These drives look very much like the old platter drives of days gone by. They came in a standardized 5.25″, 3.5″, 2.5″ and 1.8″ drive sizes. This size refers to their form factor – their physical dimensions. You can buy both SSD or older platter sized drives in these same form factors. It’s also often possible to buy display units and usb or flash drive readers in this form factor.

USB 3.0 and audio port kit that mounts in a 3.5″ drive bay

These form factors are legacy from older platter and CDROM drive dimensions; but kept because it made mounting and replacing drives easier for PC manufacturers. Desktop systems often have several 5.25″ drive bays for CDROM/Blu-ray or other optical disk formats (though back in the day, there were even 5.25″ hard drives and 5.25″ floppy drives). 3.5″ and 2.5″ drive bays can be found in desktops and laptops. As laptops shrunk, hard drive sizes did too. 1.8″ drive sizes were the smallest things got before we moved to even smaller formats that didn’t need 4 point physical mountings that spinning drives did.

  • 2.5″ Physical size: 100.5mm x 69.85mm x 9.5mm (height can vary by manufacturer)
  • Connector type: 22-pin standard 2.5” SATA connector

PCI Express Mini/Mini-PCIe/mSATA/MO-300 (full or half size)

This form factor isn’t very common for storage and is something of a halfway step between platter drive form factors and M.2. You’re likely to encounter it in laptops. This form factor comes in two varieties: full size and half size.

Great care must be taken with this form factor. Both mSATA and mini PCIe cards have the exact same form factor AND connector type – but may or may not support both product types.

  • Physical size: 50.8mm x 29.85mm
  • Connector type: 52-pin card edge connector (split into 16-pin & 36-pin sections)

Half slimSATA/SlimSATA (MO-297)

This is a rarer format these days. This form factor comes in half and full sized versions. It can be identified by the fact it uses the standard 22 pin SATA connector.

  • Physical size: 39.8mm x 54mm
  • Connector type: 22-pin standard 2.5” SATA SSD connector

M.2 – Keys and slots

M.2 is the latest, most modern form factor for SSD devices. M.2 was introduce as the Next Generation Form Factor, but I have only seen it referred to as M.2. You can find not only storage in M.2 form factor, but also WiFi, bluetooth, GPS, and other devices. This form factor comes with two important form factor parameters: length and keying.

M.2 SSDs typically come in the three sizes above, which may be deduced from the card name —2242, 2260, and 2280 – “22” represents the width in millimeters (mm), while the next two digits represent the length, also in mm.  It is possible to have a wide variety of widths and lengths – but the above sizes are the most common for storage.

Next up is their “key” type. Believe it or not, there are 12 different kinds of M.2 keying, but the most common for storage are B, M, and B+M.

In this case, you can often determine the interface type by the physical key-ing. B+M (which can fit in socks for B-keyed and M-keyed modules) are usually SATA interfaced. M.2 devices that use the NVMe interface are often only M keyed.

Interfaces

Serial-ATA/SATA

SATA is still probably the most common interface on the market today. You can find it on everything from older platter drives, SSD drives of many form factors, Blu-ray drives, CDROM drives and burners. It was a great replacement for the older IDE interfaces of the 90’s.

SATA has gone through numerous upgrades over the years as speeds have increased. Most modern drives today use SATA 3 – which delivers 600MB/s peak performance. SATA maintains very good backwards compatibility with older versions of SATA. Due to designs, most SATA3 SSD drives get 500-550MB/s. Physical spinning platter drives usually can only get to 100MB/s due to their physical speed limitations (limitations of the read heads/platters – not the interface). So just moving from a platter drive to a SSD version of the same SATA interface can often yield you around a 5x speedup.

The important point about this interfaces is to know that if your device uses the SATA 3 interface, you won’t be getting faster than 6GB/s performance. This can be confusing because some system that use the M.2 form factor supports SATA3 and the much faster NVMe interface.

You might think you are upgrading when you get rid of your 2.5″ form factor SSD drive that has the classic slimline SATA connectors (the ones shown above) for your fancy new M.2 form factor drive, but if that new M.2 drive uses the SATA 3 interface internally, you will be getting pretty much the same drive.

PCIe

PCIe drives are a little more interesting because they’re using the main interface bus of the system as opposed to through a storage protocol like IDE/SATA/NVMe. These devices typically carry additional development costs because they must write their own software/hardware layers to convert PCIe protocol read/writes to solid state memory access.

While PCIe devices have theoretical maximums that are far in excess of other drive interfaces, most typically interface via PCIe 1.x or PCIe 2.x specifications – meaning they have maximum 250-500MB/s rates. Early Intel Optane memory drives used PCIe because that interface was the only ones that could get the to the 2800MB/s range before NVMe.

NVMe

NVMe stands for the Non-Volatile Memory express interface. The Non-Volatile Memory Host Controller Interface Specification (NVMHCIS) is an open logical device specification for accessing non-volatile storage media attached via the PCI Express (PCIe) bus.

Interfaces that were designed during the era of physical platter drives (IDE/SATA/etc) have very different latency profiles and very linear/serial input/output characteristics. These interfaces weren’t designed to exploit the unique performance characteristics of non-volatile memory storage. The NVMe interface was designed to capitalize on the low latency and massive internal parallelism of solid-state storage devices.

By its design, NVMe allows host hardware and software to fully exploit the levels of parallelism possible in modern SSDs. As a result, NVMe reduces I/O overhead and implements performance improvements like multiple long command queues, and reduced latency. 

How fast is NVMe? Well, some drives advertise throughput rates up to 3500MB/s (Samsung 970 Evo Pro) – which is almost 6 times the speed of SATA 3 SSD’s. This makes them around 35 times faster than platter-based hard drives.

Putting it all together

So, now we can put these things all together to help us understand how the different combinations work; and why people often confuse performance when they are not clear about both interface and form factor

Form factorInterfaceSpeed
3.5″/2.5″/1.8″ platter-based hard driveIDE 5MB/s to 133MB/s (ATA100/133)
3.5″/2.5″/1.8″ platter-based hard drive SATA3100MB/s typical
3.5″/2.5″/1.8″ Solid State Memory hard drive SATA1/2/3 150/300/600 MB/s max
M.2 Solid State DriveSATA1/2/3 150/300/600 MB/s max
mSATA (typically SATA1)SATA1/2/3150/300/600 MB/s max
PCIePCIe 1.x, 2.x, 3.x, etc250MB/s, 500MB/s, 1GB/s, etc
M.2 Solid State DriveNVMeUp to 3500+ MB/s

Now it’s more clear why one needs to pay attention to the form factor AND interface. The form factor can give us hints as to what interface is used, but is not a sure-fired way to know the performance characteristics.

You could have a 3.5″ drive that is IDE, or a platter-based SATA3, or even a SATA3 based SSD. Each has almost an order of magnitude performance difference between the previous. A PCIe device might use PCIe 1.x and get 100MB/s or be as fast as an NVMe based M.2 drive if it’s PCIe 3.x. A mSATA/mini PCIe device might give you 150/300/600MB/s if it’s SATA1/2/3, or 250MB/s, 500MB/s, or 1GB/s if it’s PCIe.

One of the more common current difficult ones is reading advertisements that tout M.2 drives. Many do not clearly advertise the internal interface – and you can’t tell by looking at the drive or connection. As we have seen, a M.2 drive that has an internal NVMe interface might make it up to 6x faster than the same one with a SATA interface internally.

Next up:

Now that we understand form factors and interfaces, one can move on to understanding the memory technologies behind SSD drives. There are many different kinds of memory that affect performance just as greatly as interface. Most SSD’s are designed with MLC, TLC, or QLC memory configurations. Even newer is XPoint memory used in Intel’s Optane drives.

Each of these memory technologies has performance characteristics on top of the limitations of their interfaces. Some of technologies lead their drives to become slow once the drive is mostly full, some start slowing when the drive is even half full. Some have longer MTBF reliability while others statistically fail much earlier. In some cases, different controller hardware can do much better or worse jobs with these inherent limitations.

But that might be a talk for another article…

Resources:

Interfaces:

Form factors:

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