History of SLC-MLC-TLC and What to Expect From QLC

This article was written on September 23, 2019 by Hubbert Smith, guest blogger on SoothSawyer and board member of SNIA.

History

• SLC – single level cell, stores a single bit, a cell uses 2 charge levels, a full charge 1.8V is a one and zero charge is a zero
• MLC – multi level cell stores two bits, a cell uses several charge levels to store 2-bits, 1.8V, 1.2V, .6V, and 0V
• TLC – three level cell stores three bits, a cell uses more charge levels, storing 3-bits, 1.8V, 1.5V, 1.2V, .9V, .6V, .3V, 0V
• QLC – quad level cell stores four bits, a cell uses even more charge levels … you get the idea. Voltage charge deltas are even tighter and more error prone.

Now we look at capacity and the value of the NAND and SSD.

• as compared to SLC, MLC doubles capacity, adds 100% more bits per cell.
• as compared to MLC, TLC adds 50% more bits per cell.
• as compared to TLC, QLC adds only 25% more bits per cell.

Diminishing returns?
QLC is only 25% better capacity than TLC. and with every gen the industry trades slower and slower performance with poorer write endurance … for just 25% better capacity, definitely diminishing returns.

Additionally NAND is ugly and QLC NAND is whole new level of ugly
NAND cells are less than perfect, firmware goes thru all sorts of contortions to identify and correct media errors. As mentioned earlier, SSD data retention relies on the electrical charge of a cell. And given enough time, these electrical charges will evaporate. so recharging cells is one of the many maintenance tasks handled by firmware. When SSD is plugged in, firmware will refresh cell charges each 30 to 60 days. So what happens when SSD is without a power source to refresh cell charges? … no power means no cell recharge. Sooner or later the electrons will drift away and the cell electrical charge will evaporate; and data loss occurs

Promise of QLC very different than reality of QLC

• There are systems vendors attempting to drive to lower cost/GB, in a system, this is likely workable.
• There are memory vendors attempting to use QLC for cold storage, these folks are naively over-selling QLC.

Humble advices:

• Stick to proven TLC; let someone else save a nickle and learn hard lessons.
• Consider SLC for systems where the data sets are small but the overwrites are high. (Its endurance is far higher than that of MLC, TLC and QLC)