Vendors of solid state drives (SSDs) to the consumer have switched cell size and that’s causing the amount of useable storage space to fall.
In January, many vendors moved to 2x (24.25) nm 2 bit per cell flash, compared to 3x (32,34) nm 2 bit per cell flash – because of price, the latter is more expensive than the former.
While both these types of flash give about 10,000 reliable erase write cycle, the standard Sandforce 1222 operates with eight percent over provisioning for failures, giving a useable 120GB from 128GB.
With the cheaper flash, however the soft error rate is much larger, and only gives around 115GB of useable space, because of 15 percent overprovisioning.
There is a way to fix this problem by using the Sandforce 2281/2282 controller but so far it’s not shipping in a drive aimed at consumers. Presumably, again, because of cost. OCZ, to the best of our knowledge, is the only vendor to have acknowledged the problem and pledges to fix it.