SSDs are fast storage devices that have come down in price a lot over the years. What you may not know about is the clever use of technology that has been used to conceal the fact that this price drop has actually come at other significant costs. Before we discuss the different types of flash, it will be helpful to cover three more acronyms.
Warranty acronyms
SSD warranties typically have two values, a number of years, and a TBW. It stands for TeraBytes Written and is a measure of how many Terabytes the drive manufacturer guarantees the drive to be safe to write. This value always varies depending on the size of the drive, so sometimes you might also or alternatively see the TDW rating. This stands for Total Drive Writes and is – generally – the same across all capacity variants in a series of drives as the rest of the components are the same.
For example, the Samsung 980 Pro series comes in four capacities 250GB, 500GB, 1TB, and 2TB. These respectively offer 150, 300, 600, and 1200 TBW warranties. As the size of the drive doubles, so does the warrantied amount of data written to the drive. This comes out to 600 full drive writes or 600 TDW covered under warranty.
The final acronym is DRPD or Drive Writes Per Day. This is just a measure of how many of the warrantied total drive writes you can fit in the warrantied time frame. Again, for the Samsung 980 Pro, the warranty period is 5 years, in which there are 1826 days. That means you could write about a third of the drive’s capacity, every day, for the full 5-year warranty period. At the 250GB capacity that’s “only” 83GB a day, for five years. At the 2TB capacity that’s 666GB, every day, for five years.
SLC
SLC flash memory is the type of flash used in the first SSDs and stands for Single-Level Cell. It stores a single binary bit of data in each memory cell. While the performance and longevity of early SSDs were low, this was due to the lack of maturity in the market. Those issues have now been resolved with much faster connectors, better drivers, and better wear levelling technology.
SLC is the fastest type of flash memory and has the longest lifespan. It is, however, the most expensive, and is basically not used anymore. SLC is essentially only found in enterprise/data centre environments where reliability is key, and the cost is much less of an issue.
MLC
MLC stands for Multi-Level Cell. This type of flash memory stores two bits of data per flash memory cell. This type of memory wasn’t named with any sort of forward-looking mind as you might be able to guess from the names of the other types of flash. The big difference here is that by doubling the amount of data that can be stored in each memory cell the overall capacity of any similar drive is doubled. Amazingly, this doesn’t require any hardware change at all to the underlying flash memory, only to the SSD controller, so the price per gigabyte of storage can essentially be instantly halved.
The issue with storing two bits of data per cell is that it gets harder and slower to both read and write them. Additionally, the overall lifespan of the memory cell is reduced. With MLC performance is still pretty solid, and while lifespan is reduced, simultaneous improvements in other areas, including wear levelling made up for this. There are a very small number of MLC drives still currently available, while they are cheaper than SLC, they’re still not cheap and have been superseded.
TLC
TLC stands for Triple-Level Cell. It is the current standard memory configuration for flash memory in SSDs. As you might be able to guess from the name, it stores three bits of data per memory cell. That’s triple the storage density of SLC and 50% higher density compared to MLC. TLC SSDs currently cost around twice as much as a comparably sized HDD with the price sweet spot being 2TB.
Again, TLC comes with the same drawbacks and mitigating factors as well as a few new improvements. It suffers from a lower life span than MLC flash, but improved wear levelling helps to manage this. Especially when combined with overprovisioning, which is including some extra memory cells that the SSD controller can choose to swap in for older worn cells.
TLC is also slower to read and write to. Read speeds aren’t an issue, even with modern high-speed storage mechanisms, they can still saturate the bandwidth. Write speeds are an issue though, and these have mostly been mitigated through the use of an SLC cache. As we covered here, SLC caching involves treating part of the TLC as SLC which can be written to a lot faster. The data is then offloaded to the TLC as fast as possible to free up space. In high-end TLC SSDs, this cache is typically a third of the remaining storage space which can be up to 600GB on an empty 2TB drive.
Note: The Samsung 980 Pro, for which we provided stats above, uses TLC flash.
QLC
QLC stands for Quad-Level Cell. Predictably it stores four bits of data per cell offering four times the storage density of SLC and double the density of MLC. It follows the same trends as the rest of the market, being slower to write to, read from, and having a lower lifespan than TLC flash.
There are a relatively small number of QLC SSDs on the market. While they do offer cheaper prices at similar capacities, and larger capacity models, they aren’t that much cheaper than TLC. They also come with even more severe speed penalties when the SLC cache is exhausted. The technology will continue to mature and over time may replace TLC as the standard storage method.
While not directly comparable to the 980 Pro, the Samsung 870 QVO uses QLC and comes with a 3-year warranty or 360TBW per terabyte. It comes in 1, 2, 4, and 8TB capacities rather than topping out at 2TB. The longevity decrease can be fairly easily seen in high-end drives that are as closely comparable as possible. With more levels per cell, you get a lower TBW and TDW rating. Accordingly, the warranty period may be reduced to keep the DRPD rating up at marketable values.
PLC
As you might be able to guess, PLC stands for Penta-Level Cell. It stores five bits of data per cell. There are no commercially available PLC SSDs currently as this technology is still in development. It will offer the same pros and cons. Increased storage density and reduced price, at the cost of reduced lifespan and speeds. Memory manufacturers are also talking about possibilities beyond PLC including storing seven levels per cell. These will likely take some time to come to market as other technologies will need to improve or be invented in the meantime.
Conclusion
SLC, MLC, TLC, QLC, and PLC, are all essentially the same physical thing. Each memory cell is a single transistor. While the technology has evolved over time these have changed somewhat, the basic raw functionality and method behind each of them is identical. The main difference is in how they’re controlled. In the real world, its easy enough to simplify the situation to fewer levels per cell are faster but more expensive. As long as you’re aware of the different levels in the products you’re looking at, and the overall state of the market, you can make an informed decision.
TLC is currently the way to go unless you really need an 8TB SSD. With that, if you’re looking at speed as a key factor, look for drives larger drives with large SLC caches. Often the SLC cache may be mentioned, but not have its capacity advertised. It’s important to find trustable and detailed reviews that check for this if high-performance is what you’re aiming for.
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