Is TLC NAND flash memory so slow?

Source:   Editor: Co Co Update Time :2019-10-06

Is TLC NAND flash memory so slow?   

There are three types of NAND Flash , including Single Level Cell(SLC),Multi Level Cell(MLC)and Triple Level Cell(TLC). SLC NAND flash are the best both in performance and reliability, but the cost is the highest. MLC NAND flash are second in performance and reliability, having a balance in performance, reliability and cost. TLC NAND flash was brought it into the SSD market by Samsung only after 2012, previously used mainly in USB and memory card. After two years of Samsung's lead, other manufacturers are finally catching up it and a large number of TLC SSDs are on the market.TLC Flash can store more data in the same area than MLC while MLC can store more data than SLC. In short, TLC is a kind of relatively cost-effective flash memory.

Except for very few SSD with MLC NAND flash and QLC NAND flash, the SSDs we buy today are installed with TLC NAND flash. Today, let's introduce some lesser-known TLC secrets.

Why is TLC write speed slow?
Flash memory records and expresses data by storing electronics. The electronics are stored in a part called Floating Gate. By exerting a reference voltage to the Control Gate and judging whether the source and drain are conducted, then it can be determined whether there are electrons stored in the Floating Gate, so as to realize data read in flash memory.

Toshiba invented NAND flash memory in 1984, the earliest NAND belonged to SLC, which means each storage cell could record 1bit of data. For SLC flash memory, FT Floating Gate with an electron is for 0 and without electron for 1.

The flash write process is actually converting a portion of 1 to 0, by "filling the FT floating gate" with electrons. The erasure process involves " releasing " all the electrons in the gate, making them back to 1.

The floating grid is“charged”in stages by gradually increasing the “threshold voltage”, each stage being compared with the “Reference Voltage”—— a predetermined dividing line defied by different data. A storage cell in the SLC only needs to distinguish between 0 and 1, thus only one reference voltage is required.

When MLC expresses 2 bits of data, 4 states as 11,01,00 and 10 need to be differentiated with the application of 4 reference voltage. Writing process of MLC therefore is much slower than SLC.

In the TLC, each storage cell needed to record 3 bits of data and binary information increase to eight. Seven reference voltages needed to isolate the eight states. The write process of the TLC also requires more comparison and confirmation, so the writing speed decreases even lower.

Why there is only SLC cache, but no MLC cache?
PSLC mode is a technology using part of TLC flash cell as SLC, thereby greatly improving the short-term writing speed. At the same time, using TLC temporarily as MLC is called pMLC mode. pMLC mode is rarely mentioned , as it doesn’t seem as cost-effective as pSLC. Here is the data metrics of a certain flash memory in different modes.

Though simulating TLC to more advanced flash memory reduces reading and writing time, each flash memory unit accommodates less data, overall,pMLC is even slower than TLC in reading and writing. Nevertheless, pMLC is less error prone, and can be used in some industrial applications for flash controllers with low error correction capability.

Does 3D stack solve the TLC problem?
There are many problems in traditional 2D TLC flash memory, of which some can be solved. For example, poor write performance can be improved by SLC Cache. As long as the capacity of cache is large enough, users will not feel the slow write speed. In addition, if SLC Cache is used properly, the lifespan of SSD will be extended.

However, there are still some problems to be solved. The amount of charge stored in per cell decreases and adjacent storage cells also produce charge interference after traditional 2D NAND flash reaching a certain density. Interference between cell is more serious after the 20nm process was lunched. If the data have not refreshed for a long time, the speed of reading old files will drop like the previous Samsung 840 Evo.

Instead of shrinking cell, 3D NAND flash encapsulates more cell through 3D stacking technology, which can also achieve more capacity.

In the past, 2D NAND flash used FT floating grid layer as the electron container, while the main 3D NAND flash memory using CT charge trap to store electrons. The following is a structure comparison of flat and Toshiba BICS 3d flash memory.

As BiCS structure adds space between storage cells, amount of data in a single programming sequence can be increased to improve writing speed. At the same time, error rate reduces due to lower read/write interference,BiCS also leads to higher write/erase cycles or “write life”as we call.

The increase in TLC write speed can also be upgraded by structural improvements

Short board of TLC write speed mainly rely on improving concurrency to achieve. Currently, a flash memory die usually has 2 Planes.

Toshiba plans to double the speed of flash memory writing by increasing the number of planes to 4 in its next-generation BiCS5 96-layer stacked 3D flash memory. As technology keeps improving, the performance of TLC will be better.

Finally, let's talk about purchasing opinions about TLC SSD
For TCL NAND flash, what we concern most is the lifespan. In fact, as long as the TLC NAND flash is produced by original factory, the P/E and lifespan of it are guaranteed. For example, Toshiba Q300 and OCZ TRION 100 use stable enterprise eTLC with longer service life. Samsung 3D NAND flash lasts much longer than the normal TLC NAND flash. So, it is unnecessary to worry too much about the lifespan of SSD.
As for performance, I believe that you buy TLC SSD is not to pursue high performance. In terms of performance, TLC SSD can not rival most MLC SSD. But performance of TLC SSD is much better than HDD. It's absolutely no problem to replace HDD with SSD as the system disk.
Considering the cost-effective, now the various TLC SSDs are actually quite good.

The comparison between SLC, MLC and TLC

What is TLC granule? What is the difference between it and MLC and SLC?

Refusing to Cheat: How Does the Hundreds of Erasing Lives of TLC Flash Memory Become 3,000 Times?