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SLC, MLC or TLC NAND for Solid State Drives ?

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SSDs (Solid State Drives) use NAND flash chips. Each of these chips contain millions of cells with limited number of write cycles. There are different types of NAND flash chips in use today with different characteristics as follows:

SLC (Single Level Cell) - highest performance, at a very high cost, enterprise grade NAND
~ 50-100k P/E (Program/Erase) cycles per cell, highest endurance
- lowest density (1 bit per cell, lower is better for endurance)
- lower power consumption
- faster write speeds
- much higher cost (3+ times higher than MLC)
- good fit for industrial grade devices, embedded systems, critical applications.

eMLC (Enterprise Multi Level Cell) - good performance, aimed at enterprise use
~ 20-30k P/E cycles per cell, great endurance
- high density (2 bits per cell)
- lower endurance limit than SLC, higher than MLC
- lower cost
- good fit for light enterprise use and high-end consumer products with more disk writes than MLC.

MLC (Multi Level Cell) - average performance, consumer grade NAND
~ 5-10k P/E cycles per cell
- higher density (2 or more bits per cell)
- lower endurance limit than SLC/eMLC
- lower cost (3 times lower than SLC)
- good fit for consumer products. Not suggested for critical applications which require frequent updates of data

TLC (Three Level Cell) - lower performance, lowest cost NAND
~ 3k P/E cycles per cell
- highest density (3 bits per cell)
- lower endurance limit than MLC and SLC
- best price point (30% lower than MLC)
- somewhat slower read and write speed than MLC
- good fit for lower-end consumer products. Not recommended for critical applications which require frequent updating of data

3D Vertical NAND - newer TLC NAND with different architecture, larger geometry and much higher endurance than TLC
~ 3-10k P/E cycles per cell
- faster and more reliable than TLC
- uses less power than TLC
- performance and P/E cycles comparable to MLC

QLC (a.k.a. 4-bit MLC) - newer, 4-level MLC with greater data density for larger SSDs
~ 1k P/E cycles per cell
- great cost per gigabyte in large capacities
- sustained write speeds are low when cache exhausted
- sensible

Generally, SLC and eMLC drives are traditionally the fastest, most reliable and most expensive drives available, usually used in the enterprise because of their considerably higher cost. MLC, TLC and 3d TLC NAND are widely used consumer grade memory, with MLC being slightly better in terms of endurance. Newer 3D NAND TLC is comparable in performance to MLC drives, with even better price point. QLC (quad-level NAND cells) is another newer technology with great price for larger drives, possibly better suited for storage drives.

The move from 34nm to 25nm (and then to 19nm) has generally reduced the NAND PE lifespan - on 34nm each cell was rated for 5-10k PE cycles, on 25nm this drops down to 3-5k. On 19nm TLC NAND it varies a lot, but it can drop down to as low as ~1k estimated PE cycles.

NAND Endurance ratings for common/older SSDs:
ADATA (S511) - 5k PE cycles, 25nm
Corsair (Force 3/GT) - 3k PE cycles, 25nm
Corsair (Performance 3) - 5k PE cycles, 32nm
Crucial (M4) - 3k PE cycles, 25nm
Intel (SSD 320) - 5k PE cycles, 25nm
Intel (SSD 510) - 5k PE cycles, 34nm
Kingston (HyperX) - 5k PE cycles, 25nm
Mushkin (Chronos) - 5k PE cycles, 32nm
OCZ (Agility 3, Vertex 3, Solid 3) - 3k PE cycles, 25nm
Patriot (Pyro) 3k PE cycles, 25nm
Plextor (PX-128M2S/P) 5k PE cycles, 32nm
Samsung (SSD 830) 5k PE cycles

Notes: Larger TLC/QLC drives may yield similar longevity as smaller MLC drives, considering you can average out the wear over higher number of cells.
Over-provisioning (OP) is sometimes used to increase drive endurance by setting aside free space that is inaccessible by the user for controller swap space.
To improve SSD endurance, one can leave at least 10-20% of free space to simulate over-provisioning. This can also be achieved with formatting to a lower capacity.
Even with low PE cycles NAND, endurance of most modern SSDs is over 10 years of typical use.

  User Reviews/Comments:
by anonymous - 2014-12-19 12:49
"level", not "layer"

"layer" has a meaning for 3D (V-NAND, etc.)
by anonymous - 2015-04-03 14:32
wrong information, please look into this link and other studies done comparing MLC to SLC.
by Philip - 2015-04-03 16:42
The information is correct. The article you quoted actually compares MLC to TLC, and it also confirms that by design TLC flash is more sensitive to wear because it has more cell states.

In addition, those tests only indicate how many times data can be written, but not how long the written data can be retained, so they give a useful, but by no means complete picture.
by Petru Mateescu - 2016-04-24 12:36
I also think that the info is incorrect.

Let's compare 2 SSD:

#1-----ADATA PREMIER PRO SP5050 240GB(a TLC)
#2-----KINGSTON SSDnow V300 240GB(a MLC)

#1 has an advantage of around 100 mb/s

by Anonymous - 2016-05-05 11:56
The article is accurate.

ADATA SP550 is a TLC drive, with an SLC cache (2-8GB).

So it is kind of like a hybrid SSD/HDD drives where the initial writes are done to the cache (SLC layer), which are super fast, up to 500MB/s, but when you do a large transfer of, say, 20GB, once the cache is filled, it slows down to the TLC speeds which can be slower than HDD (SP550 120GB drive's sustained average write speed is 73MB/s on a 26.8GB transfer, which is abysmal).

Check out tweaktown's review of the SP550. A lot of the SSD manufacturers are switching to cheaper TLC drives and by using a small SLC cache, they get to advertise "up to 500MB/s transfer speeds". The key being the "up to" part.

It is very deceiving, really. Especially when a company like Crucial releases the next version of a super popular SSD but switches from MLC to TLC in the process, while still advertising the same speeds. In reality, the BX200 is much slower than the BX100 when sustained writes are taken into consideration.
by Anonymouse - 2016-09-13 11:48
"by Petru Mateescu - 2016-04-24 12:36

Googling 'v300 nand switch' will provide you with your answers. Kingston tends to use some shady behavior with their lower end products and blame the consumer."
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