With Flash Memory Summit approaching next week, I thought it would be a good time to dig into the technology and life cycle of the SSD. Unlike traditional hard drives, data storage data in SSDs is not on a magnetic surface, but rather inside of flash memory chips (NAND flash). By design, an SSD is made by a motherboard, a few memory chips (depending on the size in GB of the drive) and a controller which commands the SSD.
The memory of SSDs is a non-volatile memory, in other words it’s able to retain data even without power. We can imagine the data stored in the NAND flash chips as an electric charge preserved in each cell. With that in mind, the question arises: how long is the life span or life cycle of a SSD?
The wear and tear of flash memory
It is known that the writing operations wear out the memory cells of a SSD, reducing its life. But will the memories wear out all in the same way?
The memory used in flash chips is not all the same, there are actually three types of NAND:
SLC (Single Level Cell) – 1 bits of data per cell
MLC (Multi Level Cell) – 2 bits of data per cell
TLC (Triple Level Cell) or 3-bit MLC – three bits of data per cell
You can see: The more levels a cell has, the more storage space bits have in a cell, resulting in the production of higher capacity chips. Thanks to the technological advances of today we have SSDs which are able to store several GBs and are at an affordable price. No wonder that a recent report shows that the TLC memory type should equal about 50% of total NAND chips by the end of 2015, with a cost of production of about 15% – 20% less compared to MCL chips.
However, there is a downside: Adding more bits to the cells reduces their reliability, durability and performance. It is quite easy to determine the state (how much space it has) of a SLC cell, as it is either empty or full, while it is more difficult to do the same for MLC and TLC cells as they have multiple states. As a result a TLC cell requires 4 times the writing time and 2.5 times the reading time of a SLC cell. When discussing the life cycle of an SSD, storing multiple bits per cell also means to speed up the wear of the NAND memory.
A memory cell is made by a floating-gate transistor. It consists of two gates, the Control gate and the Floating gate insulated by a layer of oxide (you can see a schematic representation on the right). Each time operations are performed, e.g. programming and erasing the cell, the oxide layer that traps electrons on the floating gate wears. Consequently, as the oxide layer is weakened an electron drain from the floating gate may occur.