Understanding TLC NAND
by Kristian Vättö on February 23, 2012 1:14 PM EST- Posted in
- Storage
- SSDs
- OCZ
- Indilinx Everest
- TLC
Our thoughts are a bit mixed. On the one hand, cheaper SSDs are exactly what consumers want. The performance is still there compared to hard drives, no matter what NAND is used. If you go to an Apple Store today and try out MacBook Air and Mac Pro, the MacBook Air will often feel faster, even though it's the slower Mac in terms of processing power. This is solely due to the presence of an SSD. An SSD can bring new life to a computer that is otherwise considered obsolete. That's why we think everyone would want an SSD, but it's understandable that the masses won't adopt SSDs until the price and capacities are reasonable. This is definitely where TLC shines—it provides us with noticeably cheaper SSDs, possibly cheap enough for the masses to adopt (e.g. well under $1 per GB).
On the other hand, we're concerned that the cut in prices is done at the expense of endurance. One advantage often heard about buying an SSD is that SSDs are a lot more reliable than hard drives. In terms of P/E cycles, that is probably true with current MLC NAND. However, there have been quite a few widespread firmware issues, such as SF-2281 BSOD and Intel 320 Series 8MB bugs. Those have been fixed, and we may finally be looking at SSDs which have good performance, adequate endurance, and are more or less trouble-free. However, TLC will require new controller logic, and new logic may result in additional firmware issues.
The earliest SSDs lacked performance, even though they were faster than most hard drives, especially in seek times. In just a few years, performance has increased exponentially, maybe even to a point where the average user won't notice the difference between the fastest SSD and a mediocre SSD.
Given the desire for performance, reliability, and cost, TLC NAND may take away one from the triplet: endurance. Notice we said "may", because P/E cycles aren't everything. It has been claimed that algorithms to minimize write amplification will follow Moore's Law, just like NAND does. In other words, every time there is a die shrink, wear leveling has been improved in order to keep endurance the same. On top of that, improvements in manufacturing technologies can keep the P/E count up as well. 20nm IMFT MLC is claimed to have 3000-5000 P/E cycles, just like 25nm IMFT MLC.
The good news is, MLC NAND will stay in production and hence MLC NAND based SSDs are not going anywhere. What TLC will provide is freedom of choice. If you use your computer for checking email and browsing the Internet, no doubt a TLC based SSD will be sufficient. For the majority of consumers, TLC SSDs should meet their demands.
In addition, the SSD market is evolving quickly; if you buy the best SSD today, it won't be the best for very long. Let's say that it lasts you for four years. In that time, the SSD market will change a lot—four years ago, we were looking at 16GB SSDs for nearly $600! By the time a typical SSD is ready for replacement, you will be looking at much faster SSD with more capacity, and likely for a lower price. In 4.5 years, we have gone from that 16GB offering with performance that often trailed behind contemporary HDDs to 120GB SSDs that are up to a couple orders of magnitude faster than HDDs on random access patterns (and still several times faster for sequential tranfers), all for a starting price of around $170. If that pattern holds for the next four years, we'll be looking at ~1TB SSDs in four years that offer transfer rates that would saturate multi-lane PCIe interfaces at even lower prices. While we expect the rate of progress to be quite a bit slower over the next four years, there's still plenty of room for improvements in SSD technology.
As far as TLC-based SSDs are concerned, all we can do now is to wait for the first product announcements to come. Once we get some review samples, we'll be sure to put them through our SSD test suite and see how they stack up to existing drives.
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ionis - Friday, February 24, 2012 - link
Both paragraphs you quoted state that TLC has 8 states."TLC takes that a step further and stores three bits per cell, or eight voltage states (000, 001, 010, 011, 100, 101, 110, and 111)."
"With MLC, there are four states, and eight states with TLC. "
JMC2000 - Friday, February 24, 2012 - link
That is the total number of voltage states per cell, i.e.:1 bpc = 2 voltage states per cell (2^1)
2 bpc = 4 voltage states per cell (2^2)
3 bpc = 8 voltage states per cell (2^3)
The voltage states are what allow each bit to read as 0 or 1. TLC has 8 voltage states to allow intermediary changes in the values of the three bits in each cell: 000, 001, 010, 011, 100, 101, 110 and 111.
ionis - Friday, February 24, 2012 - link
Yes. That's the argument. So it should go 16->32->64. Where is the 48 coming from?P.S. we're running out of space in this thread!
Andunestel - Friday, February 24, 2012 - link
The commenter above explained.It's not double each time. The number of combinations, or voltage states, increases exponentially with the number of binary digits represented.
SLC 1 bit (0,1) = 2 states
MLC 2 bits (00,01,10,11) = 4 states
TLC 3 bits ( 000,001,010,011,110,111) = 6 states
Notice that MLC is 100% > SLC, but TLC is only 50% > MLC?
In other words:
SLC = 16GiB
MLC = SLC x 2 = 32 GiB
TLC = SLC x 3 = 48 GiB
- or -
TLC = MLC x 2.5 = 48GiB
Taracta - Sunday, February 26, 2012 - link
For TLC you have left out 100 and 101 so you would haveTLC 3 bits (000, 001, 010, 011, 100, 101, 110, 111) = 8 STATES!
The information is stored per CELL and the same cell is used for SLC, MLC and TLC. The difference between them is the amount of bits per cell that is all. It is not 1 MLC = 2 SLC or 1 TLC = 3 SLC. It is if cell has:
2 electron states = SLC
4 electron states = MLC
8 electron states = TLC
In binary these states are represented by:
2 states = 1bit
4 states = 2bits
8 states = 3bits
This Guy - Friday, February 24, 2012 - link
Your confusing bits with data. Let's look at this problem in decimals.If you have one symbol between 0 and 9, you can represent any number 0-9.
If you have two symbols, 0-9, you can represent any number between 0-99
If you have 3, you can represent 0-999
BUT you still only have three symbols.
Back to binary, a SLC stores 1bit, MLC 2bits and a TLC 3bits. So if you have 3 SLCs, you have 3 bits and 8 possible states. Exactly the same as one TLC. I'll expand this to make this point clear:
# Cells | # Bits | # Bits | # States
6 SLC = 6x1bit = 6bits = 2^6 states
3 MLC = 3x2bit = 6bits = 2^6 states
2 TLC = 2x3bit = 6bits = 2^6 states
All three configurations can store the same data. So to answer your question, the logical blocks which SLC, MLC and TLC apear to be based on have sixteen cells per block. Hence:
16 SLC = 16 x 1bit = 16 bits
= 2^16 states
16 MLC = 16 x 2bit = 32 bits
= 2^32 states
16 TCL = 16 x 3bit = 48 bits
= 2^48 states
I know this was long and tedious, but if I'm not going to recheck this tread and I wanted to make sure I gave enough information that most people reading this should be able to understand the difference between bits and data.
Taracta - Sunday, February 26, 2012 - link
You do notice that in your decimal example that it is increasing by powers of 10 so why in your binary exapmple it is not increasing by powers of 2?16 SLC = 16 x 2^1 bit = 32bits
16 MLC = 16 x 2^2 bit = 64bits
16 TLC = 16 x 2^3 bit = 128bits
No, your exapmples are incorrect so you just further confused the issue.
KitsuneKnight - Sunday, February 26, 2012 - link
SLC doesn't have two bits, it has one. It's not 2 raised to the blah, it's just blah. Same issue applies to your MLC & TLC examples.SLC can _represent_ two values, 'on' or 'off'. MLC can represent 4 values ('on' or 'off' | 'on' or 'off'). And, likewise, TLC represents 8 values ('on' or 'off' | 'on' or 'off' | 'on' or 'off'). As you might notice, each grouping of 'on' and 'off' is a single bit.
His examples are completely correct.
JMC2000 - Friday, February 24, 2012 - link
(What I would give for an EDIT function)I forgot to add that if you had 8 bits per cell, you would have 256 voltage states (0 or 1 for each bit, plus the different variations of 8 on or off bits), though I will not list all possible combinations, as it would take too much time/room.
Taracta - Sunday, February 26, 2012 - link
I completely agree with you. The whole premise of the article is being based on the incorrect graph while having in places, the correct information in the article. There is a difference between place holders and values. SLC - 1 bit place holder 2 bits stored, MLC - 2 bits place holder 4 bits stored and TLC 3 bit place holder 8 bits stored.