SSD Form Factors Proliferate At Flash Memory Summit 2018

Last year, Intel and Samsung proposed new form factors for enterprise/datacenter SSDs with the goal of overcoming the shortcomings of the existing M.2, U.2 and PCIe add-in card form factors. Samsung’s NF1 form factor (previously known as NGSFF) had little presence at Flash Memory Summit this year without Samsung’s participation in the show, but the derivatives of Intel’s Ruler design were all over the place. Unfortunately, the Ruler has spawned not just one industry standard, but a whole family of new form factors.

Working with SFF, the committee originally formed to standardize the 2.5″ hard drive form factor, Intel’s Ruler has led to the Enterprise and Datacenter SSD Form Factor (EDSFF) family of standards.

Why new form factors?
The existing form factor options for enterprise SSDs have proven inadequate for datacenter needs. It is increasingly common for servers to use several types of SSD (boot drive, performance tier, capacity tier), and that usually requires using more than one SSD form factor. Each has its own downsides:

2.5″ SATA, SAS, U.2: Drives with the same 7mm thickness that consumer SATA drives use are relatively limited in maximum PCB area for NAND flash packages, and internal volume for power loss protection capacitors. Increasing the thickness up to 15mm allows for bulky capacitors and two PCBs stacked inside the drive’s case, but this severely compromises the ability to cool the drive. Backplanes for 2.5″ drives tend to be a severe airflow obstruction.

PCIe add-in cards: Half-height half-length (HHHL or MD2) cards have plenty of PCB surface area for large amounts of flash and heatsinks that can handle 40W or more. Full-height cards increase these limits even more. This is the only current option for PCIe x8 or wider interfaces. Hot-swapping is possible with many cards, but this does little good when the cards are not accessible from the front of the server.

M.2: Enterprise SSDs typically use the M.2 22110 card size that is longer than the 2280 card used by client/consumer drives. The extra space allows for higher drive capacities or power loss protection capacitors. However, the power and thermal limits are still severely constraining. Delivering more than 8W with only a 3.3V supply requires careful system design to ensure that enough current can be provided without the voltage dropping out of the required 5% tolerance. Dissipating 8-12W usually requires heatsinks that detract from the density advantage of such a small form factor. Hot-swapping is only possible by installing M.2 drives in some form of carrier module that further inflates the space occupied by each drive.

The SFF-TA-1002 Connector
The EDSFF family of SSD form factors share a common connector standard, which has also been adopted by the GenZ interconnect and the Open Compute Project’s latest NIC form factor. The SFF-TA-1002 connector standard defines a multi-lane card-edge connector with a much smaller contact pitch than PCIe add-in card slots. Three sizes are defined, allowing for PCIe x4, x8 and x16 links, and narrower sockets can accept longer cards thanks to the notches in the card edge connector. The connector provides adequate signal integrity for data rates of at least 56 GT/s with a NRZ encoding (as used by PCIe) so it is more future-proof than most implementations of the existing connector standards for PCI Express signals. The smallest 1C variant allowing for PCIe x4 and power is 23.88mm wide, about the same size as a M.2 connector. The widest 4C variant supporting PCIe x16 links is 57.02mm, compared to about 89mm for a standard PCIe x16 slot.

Leave a Reply

Your email address will not be published. Required fields are marked *