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Introduction to SSD. Part 3. Form factor



In the previous parts of the Introduction to SSD series, we talked about the history of the appearance of disks and interfaces with drives. The third part will introduce the reader to modern disk form factors.


Solid state drives are devoid of moving parts, and the data is stored in microcircuits, which can be located on the boards with almost no restrictions. This feature of the SSD "free hands" to drive manufacturers and allows you to go beyond the usual formats.

“Classic” SSD Form Factor



The 2.5 ″ form factor was proposed by PrairieTek in 1988 and was later enshrined in the EIA / ECA-720 standard. Such drives can be connected via either SATA or PATA, although the latter are not very common. Discs of this form factor are 100 mm long, 69.85 mm wide and 5 to 19 mm high.


Speaking of the 2.5 ″ form factor, one recalls involuntarily his older brother - 3.5 ″. Solid state drives in this format are rare, but exist to this day. For example, ExaDrive from Nimbus Data. The drive boasts incredible storage capacity: 100 TB in one 3.5 ″ drive.


The 2.5 ″ also has a younger brother: a 1.8-inch form factor. This form factor is used to connect mSATA and was common in laptops.

U.2



U.2, also known as SFF-8639, was developed in December 2011 by the SSD Form Factor Working Group. The SFF-8639 standard was developed primarily for the corporate segment with support for PCIe, SAS, and SATA disks. Externally, U.2 discs differ from 2.5 ″ by another connector and a fixed height of 15 mm. On disks in the U.2 form factor, a relief bottom wall is found to improve heat dissipation.

U.2 implements three types of interfaces: SATA, SAS and PCIe. However, each connector supports only one of the interfaces. So, in a PCIe SAS backplane, the drive "will not start." This caused certain inconvenience, which another form factor had to solve.

U.3



On March 20, 2018, the Open Compute Project introduced the U.3 form factor, which solves the existing U.2 problem. According to the specification, SAS, SATA and PCIe interfaces are supported on all pins, and the interface is selected automatically, depending on the interfaces provided by the disk. U.3 drives are compatible with systems using U.2, but not vice versa.

There are currently no drives with the U.3 form factor.

M.2



This form factor is also known as Next Generation Form Factor (NGFF). The first version of the M.2 standard was released by the PCI Special Interest Group (PCI-SIG) in December 2013. This form factor is not limited to solid-state drives: there are Wi-Fi and Bluetooth-modules in this design.
Despite the fact that the M.2 device is often fixed with a screw, the M.2 interfaces support hot swapping. Thus, a hot swap is possible if the device and the motherboard support this feature.
Compared to previous form factors, M.2 offers maximum flexibility in device design. The following device specifications may vary:

  • width;
  • length;
  • height;
  • Key view and supported interfaces.

The exact size and type of key can be found by the type of device.


Add-in-card



As we learned earlier, SSDs can use PCIe lines to connect through special connectors. But there are drives that use PCIe with the original connectors. This form factor is called AiC, that is, Add-in-Card. PCIe cards vary in size.


The biggest option in the Add-in-Card is the Full-Height Full-Length (FHFL) profile. The size of the FHHL card is 120 millimeters in height and 312 millimeters in length. SSDs are usually created in a minimal profile: Half-Height Half-Length (HHHL) with a height of 79.2 mm and a length of 175.26 millimeters.

Nf1


In August 2018, Samsung introduced the NGSFF (Next Generation Small Form Factor) form factor, also known as M.3 or NF1. Samsung's form factor differs from M.2 in the increased width and lack of variety in the connectors. The length of the NGSFF disk is 110 millimeters and the width is 30 millimeters, which is equivalent to the largest M.2 board.


NF1 uses connectors identical to “M” type connectors of the M.2 form factor, however, M.2 and NF1 are not compatible with each other . The PCI-SIG does not approve of the use of the M.2 connector in this form factor, since installing M.2 devices in the NF1 connector may damage the installed equipment .

This form factor is designed for the server segment: the increased width allows you to accommodate up to 36 drives in 1U server.

Edsff



The Enterprise & Data Center SSD Form Factor (EDSFF), known as the Intel Ruler SSD, was developed by the EDSFF Working Group. EDSFF presents two versions of server SSDs: short (E1.S) and long (E1.L).

The short version of EDSFF, E1.S, is very similar to its closest competitor - NGSFF, but has a metal case that simultaneously protects the board from mechanical damage and acts as a slide for installation in the server. The dimensions of the E1.S drive are not much different from NGSFF: 111 millimeters long and 31 millimeters high.


E1.L is almost three times longer than E1.S, its length is 325 millimeters. Increasing the length of the drive allows you to increase disk capacity. In May 2019, Intel introduced the 15.36 TB SSD D5-P4326, and plans to release a model with a capacity of 30.72 TB in the future.

Conclusion


Most form factors have long been established, and all changes in drives occur “under the hood”. Nevertheless, manufacturers come up with new form factors for NVMe, with the goal of increasing the number of disks in one unit of server space.
In our Selectel Lab you can test the Intel SSD D5-P4326 15.36 TB in a server based on the high-frequency Intel Xeon W-3235.
And what do you think, how soon will a common standard for a new form factor combining the best features of NGSFF and EDSFF appear? See you in the comments!

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