NAND and cells: SLC, QLC, TLC and MLC defined

SSDs have grow to be one of the vital pervasive applied sciences during the last decade due to the unimaginable efficiency enhance they provide for desktop PCs and enterprise computing over conventional arduous disks.

They’re additionally lighter and use much less energy, so enhance battery life in laptops, and since they comprise no transferring components, dropping your gadget on a tough floor won’t harm the info on an SSD in the identical manner it will with a tough disk.

SSDs at the moment are a typical function in most new client PCs. However the know-how has some distinctive quirks that imply they must be designed, used and configured in a barely completely different manner.

SSDs have two main elements – a controller, which is a small processor with its personal reminiscence, and an array of many hundreds of thousands of flash reminiscence cells which can be used to retailer knowledge.

Flash reminiscence cells are non-volatile silicon chips (which implies they maintain knowledge when powered off, not like system reminiscence) and they are often erased and written to a number of occasions. Every cell can maintain knowledge inside a floating gate, written to with voltages.

With self-encrypting drives, that are designed so as to add a layer of strong safety to the info saved on an SSD, the controller additionally incorporates a processor devoted to encrypting and decrypting knowledge because it’s learn from the flash reminiscence.

In at present’s day and age, something you personal which serves an information storage operate is prone to comprise NAND Flash; its utilization has seen an exponential development through the years with petabytes (that’s a million Gigabyte) price of merchandise shipped over the previous decade.

These days, NAND might be discovered throughout us – the smartphone you personal, the server in a internet hosting supplier, the pc in your workplace, and even complicated medical tools may function NAND.

Not all NAND is created equally, understanding the distinction between NAND Flash varieties is a vital, but normally missed process by customers. Various kinds of NAND have completely different traits, thereby essential implications when it comes to efficiency, endurance, reliability and value.

The boundaries of flash reminiscence

The primary limitation is that flash storage stays usually dearer than arduous disks of the identical capability. Though SSD costs have come down an astonishing quantity during the last decade, partially due to elevated manufacturing effectivity with decrease overheads and explosive development in demand, there may be nonetheless an ongoing want to scale back the associated fee per gigabyte of flash storage.

The second difficulty is that flash reminiscence has a restricted lifespan. Rated in P/E (Program-Erase) cycles, every cell in an SSD can solely be written to a sure variety of occasions earlier than it not holds {an electrical} cost. There’s presently no solution to completely keep away from this drawback with any sort of flash reminiscence.


Confronted with these points, engineers have give you a intelligent solution to get extra knowledge storage capability from the identical quantity of NAND flash in an SSD – by rising the bits of information every cell can maintain. The draw back is that as every cell holds extra knowledge, it results in diminished drive endurance, decreasing the variety of P/E cycles an SSD is rated for.

NAND Flash that may solely maintain a single bit of information per cell, with two binary values – 0 or 1 – is named SLC. However this NAND is so expensive per gigabyte that SLC SSDs are prohibitively costly and never appropriate to be used in client PCs and laptops. However SLC NAND gives the very best endurance, with as much as 100,000 P/E cycles.

MLC NAND makes use of two bits of information per cell, with 4 potential binary values. When SSDs started to take off round a decade in the past, it was due to MLC NAND flash permitting SSD capacities which can be helpful in a typical desktop or laptop computer pc lastly changing into inexpensive. However the improve in storage capability reduces SSD endurance to 10,000 P/E cycles.

Whereas that diminished longevity seems like a hefty drop-off, you’re unlikely to ever “put on out” an MLC SSD, except you’re writing to it continually. In most common house setups, together with even high-end gaming, design and content material creation workstations, there are few utilization eventualities the place knowledge is written to a drive continually.

Enterprise and datacentre utilization are barely completely different, which is one cause a typical enterprise SSD is configured otherwise to a client drive, however right here too the endurance of an MLC drive ought to nonetheless by no means grow to be an issue inside even lengthy timeframes.


The idea of MLC has been so profitable, the following apparent transfer is to make use of much more bits of information per cell. 3D TLC NAND Flash makes use of three bits per cell, for 8 potential binary values, providing one other large capability enchancment / price discount. This know-how has helped SSDs grow to be mainstream, inexpensive sufficient to be used in any pc at capability ranges which can be helpful for many functions.

As of 2021, TLC has now grow to be the predominant know-how in all client SSDs, utilized in high-performing drives supposed for servers, video games PC and even probably the most demanding client-side computing, in addition to extra inexpensive entry-level SSDs geared toward extra normal use. However with TLC know-how, the endurance is diminished additional, to only 3000 P/E cycles. Which will sound like a worrying discount, however even with diminished endurance, a median consumer will discover it arduous to note any distinction between an MLC and TLC SSD.

The vast majority of software program functions spend extra time studying knowledge from a disk than writing to it. Fundamental computing duties corresponding to internet looking, e-mail and phrase processing barely write any knowledge in any respect to disk. Video games and functions are usually solely put in as soon as after which by no means write a lot knowledge once more. Even high-end duties that contain manipulating large information corresponding to video modifying and graphic design will not spend all day writing giant information – however as an alternative writes are normally sporadic.

For almost all folks, a TLC SSD is a good alternative, will carry out to a excessive customary, and prices a lot lower than a high-end MLC drive. It has now changed MLC because the dominant SSD know-how.


The subsequent step in SSD know-how is QLC – 4 bit layers per cell, for 16 potential binary values. That’s permits for one more astonishing improve in capability and discount in price. It additionally means diminished endurance, to only 1,000 P/E cycles.QLC is probably the most inexpensive NAND know-how and QLC SSDs are the most cost effective drives you should buy. Regardless of additional reductions to endurance, QLC SSDs are already changing into mainstream.

What’s 3D NAND?

As SSD know-how has advanced, engineers bumped into one other difficulty when scaling common 2D NAND with a purpose to obtain larger densities at a decrease price. In 2D NAND, the cells that retailer the info are positioned horizontally, facet by facet. Which means that the quantity of area that the cells might be positioned onto is restricted and attempting to make the cells smaller reduces their reliability. This limitation has been overcome by stacking cells vertically in addition to horizontally, permitting for larger storage capability, enhancements to endurance and decrease energy consumption. 3D NAND is now used along side each TLC know-how and QLC know-how in most high-end drives for the very best mixture of endurance, capability and value.