MLC vs TLC Storage

[Guest Storm10]

What is the difference be MLC AND TLC storage?

[NoRomanBatmansAllowed]

4 minutes ago, Storm10 said:

What is the difference be MLC AND TLC storage?

 

[Spuriae]

Realistically, implementation is more important (as is the case with most things). Even though raw MLC is faster than equivalent TLC, TLC with good caching can beat many MLC in ordinarily MLC-dominated sequential write tasks. 3D TLC beats most planar MLC for endurance. Random performance is largely determined by controller.

 

 

[mariushm]

short version:

 

Flash memory is a kind of memory that is so cheap and in such high quantity (compared to regular RAM chips) because it supports a limited number of erase cycles.

In a single Flash memory chip, data is segmented in "pages" each with 512 KB or more data, and each such page is further segmented in tiny blocks of 0.5 KB or 4 KB

You can write data in any such tiny block in any such page, but you can't overwrite the block or change the contents. You have to erase the whole page in order to write again data there.

Once a page is erased a number of times, the page becomes read only and you can no longer write data to it.

 

TLC memory stores 3 bits in each memory cell, so there's less "space" used by each bit, which also means the "quality" of the signal is lower, it takes more "processing power" to properly decode the bits when they're retrieved from memory chips and there's also better error correction required in order to correct badly read bits. Because the data is so tightly packed inside memory chips, the pages can only be erased maybe 400 to 600 times before they can no longer reliably store bits and become read only.

Because data is so tightly packed it's possible to have fast read speeds, but write speeds are often slower because the SSD controller has to work harder to figure out where to put data in such a way as to minimize the erase cycles for various memory pages.

 

MLC memory stores only 2 bits in each memory cell, so there's higher signal to noise ratio and therefore there's less error correction required and these memory chips can be erased up to 1500-4000 cycles before pages become read only. 

The number varies with the manufacturing process.. for example a 19nm MLC flash memory chip may support 3000 erase cycles while a 14 nm MLC flash memory chip may only handle 1500-2000 cycles.

 

So because of these reasons TLC memory usually is rated for lower total life  .. for example you'd see 120 GB SSD drives with MLC that have only 50 TB drive writes for their lifetime , while a similar drive with MLC memory usually is rated for 80 to 110 TB of  writes throughout its life.
 

 

[DocSwag]

MLC is 2 bits per cell, whereas TLC is 3 bits per cell.

 

This means that TLC can store more memory for the same amount of cells/cost, which means TLC based drives tend to have higher gb/$ than MLC drives.

 

However, it comes at a cost. Because of how SSDs work, this means that you sacrifice 2 times the read speed and as much as 4 times the write speed at the cell level.

 

Granted, it's not gonna be that much in practice since read and writes aren't always bottlenecked by the flash itself, but there is an impact. This article explains it a bit more deeply, albeit with SLC vs MLC (granted, MLC vs TLC is pretty much the same comparison).

http://www.anandtech.com/show/2614/2

 

However, because of all this it also means that TLC can degrade more easily, as explained here:

http://www.anandtech.com/show/2614/3

 

So basically:

 

TLC is cheaper for the same capacity than MLC, but it also performs worse and will have a shorter lifespan.