Which Form Factors for SSDs

On HDDs, it is relatively simple, form factors are generally described by a single metric, for example 2.5 or 3.5 inches, supposed to refer to the width of the drive even if the reality was sometimes different. The main component at the origin of the form factors was the diameter of the magnetic disk platters, the height varying depending on their number.

The first HDD (from IBM in 1956) used a 24-inch form factor. Five years later, Bryant Computer pushed it to 34 inches, the largest HDD never seen. But then the trend was to diminish the volume of HDDs as fast as possible. The first for personal computers was a 5.25-inch unit. This form factor was popular for many years for PCs, then for enterprise units, and has completely disappeared.

Only two form factors are now used: 2.5 and 3.5-inch with the exception of Toshiba being the last one to manufacture 1.8-inch rotational devices.

In the past, many companies entered into smaller units (1.5, 1.3, 1.0 and even 0.85 inches), but the flash technology destroyed all of them in term of cost and performance. An hard disk drive needs a minimum of components (at least one disk, one slider, one suspension, one head, one voice coil motor, one spindle motor) which means a minimum of price. With SSDs, the price roughly depends directly on the number of flash chips and consequently is lower for small capacities.

In 1995, M-Systems was probably the first in flash-based solid-state drives. But the first units that were offered came from BiTMICRO with a number of introductions in 1999 of SSDs with SCSI interface from 10GB to 18GB. The idea being to replace enterprise HDDs, the company opted for the most popular form factor at this time, 3.5-inch. There are now very few SSDs in this volume, 2.5-inch being the rule. It is also becoming the rule for enterprise HDDs. 3.5-inch remains only for high-capacity disk drives at a maximum of 7,200rpm, at 1TB and more, for PCs and nearline applications.

What has changed is the fact that SSDs are not anymore in competition with HDDs for lower capacities. So why to be restrain to 1.8 or 2.5-inch form factor when it’s possible to have more capacity in a smaller package? 0.85-inch was reached by Super Talent in 2008 (with only 4GB). For USB keys, there is no rule but one: as small as possible. For notebooks and more than that for sub-notebooks including tablets, OEMs are trying to reduce their volume, they want thinner place for the storage device and they can get it with customized flash modules.

For SSDs with mSATA or mini-SATA, utilizing SATA 2.0, the form factor is 87% smaller and 90% lighter than 2.5-inch. Viking Modular Solutions speaks about SATADIMM SSDs. Foremay is supposed to ship in 3Q11 a SATA SSD "Disk on Chip", only 22x22x1.8mm. SATA NANDrive SSDs from Greenliant are in a 14x24x1.95mm BGA package. Netlist has launched recently a mSATA slim SSD module offering up to 128GB with onboard 64MB DRAM cache. Intel and Micron expect soon 128GB flash units (with 20nm chips) smaller than a U.S. postage stamp.

Beyond that, we can anticipate future motherboards integrating directly
flash like DRAM on PC or servers. And if there is one company that can
do it, it’s Intel, manufacturer of both motherboards and flash chips.
When it will happen, many SSD makers will have difficulties to survive.

There is no more standard in the size of SSDs and the industry needs new ones. JEDEC is working on the subject.

The next trend for flash is the use of an interface that HDDs ignore but more faster than SAS or SATA: PCIe. And here the evolution is the same with the arrival of smaller mini PCIe once more to diminish the size. A standard could be appreciated and there is a new activity at NVMHCI Work Group with Cisco, Dell, EMC, IDT, Intel, NetApp and Oracle for NVM Express 1.0 specs to define an optimize register interface, command set and feature set especially for PCIe SSDs.

Conclusion: HDD and SD are different technologies that are going to follow different paths.

                          DOCUMENT:
  FIRST HDD FORM FACTOR INTRODUCTION

 
(Sources: Disk/Trend and StorageNewsletter)