Someone recently asked The SSD Guy to guess what would be the largest amount of flash that could be fit into an SSD’s case. This sounded like a fun problem, so I did a “Back-of-the-Envelope” estimate to try and figure it out.
First of all, I would judge by this post’s picture that you could get no more than 20 chip packages (4 x 5) on one side of a PC board for a 2.5″ SSD. That’s probably an optimistic estimate.
If you ignore the controller that would allow you to squeeze 40 packages onto a single circuit board.
Certain high-capacity SSDs use a “Butterfly” design to fit three circuit boards into a single 2.5″ HDD housing. With three 40-package circuit boards you could fit 120 chip packages into the 2.5″ HDD housing.
Today’s densest flash chip stores 128 gigabits or 16 gigabytes. Samsung and SanDisk can stack 16 of these chips within a single package, making a 16 x 16 gigabyte or 256 gigabyte package. SanDisk just announced a 512 gigabyte SD Card that doubles that figure, probably by using two 16-chip stacks. If we were to put 120 of these 256 gigabyte packages into an SSD case it would give us a 30 terabyte SSD.
Compare this to the 500TB raw capacity of Skyera’s highest-capacity 1U box, which was designed with an obsessive focus on getting the largest amount of flash into the box.
The 30TB SSD I guessed about above would be a “Hero” SSD. More reasonable SSDs use the same 16 gigabyte chips but only stack 4 or fewer chips in a single package. That would cut the 30 terabyte number to 7.5 terabytes, provided that the SSD used 3 internal PC boards. Since you have to put the controller somewhere then a reasonable SSD would have a smaller capacity than 7.5TB.
The 3-board design is uncommon and expensive. Most SSDs (like the one in the picture) don’t use more than a single PC board. 2TB would be a reasonable capacity for a single-board SSD. At today’s MLC prices the flash alone for a 2TB SSD would cost $800, limiting the market for such a product.
No matter whether you opt for a 2TB SSD or a 30TB SSD it’s going to cost an awful lot of money! Of course, eventually NAND flash (or its replacement technology) will change that situation, and through the same mechanism (Moore’s Law) the largest SSD that can fit into a 2.5″ HDD case will grow as well. But for the time being those two numbers are about the limit of what can reasonably fit into this industry-standard 2.5″ HDD form factor.