Nearly all consumer SSDs use a memory technology called NAND flash memory. The write endurance limit is due to the way flash memory works.
Put simply, flash memory operates by storing electrons inside an insulating barrier. Reading a flash memory cell involves checking its charge level, so to retain stored data, the electron charge must remain stable over time. To increase storage density and reduce cost, most SSDs use flash memory that distinguishes between not just two possible charge levels (one bit per cell, SLC), but four (two bits per cell, MLC), eight (three bits per cell, TLC), or even 16 (four bits per cell, TLC).
Writing to flash memory requires driving an elevated voltage to move electrons through the insulator, a process which gradually wears it down. As the insulation wears down, the cell is less able to keep its electron charge stable, eventually causing the cell to fail to retain data. With TLC and particularly QLC NAND, the cells are particularly sensitive to this charge drifting due to the need to distinguish among more levels to store multiple bits of data.
To further increase storage density and reduce cost, the process used to manufacture flash memory has been scaled down dramatically, to as small as 15nm today—and smaller cells wear down faster. For planar NAND flash (not 3D NAND), this means that while SLC NAND can last tens or even hundreds of thousands of write cycles, MLC NAND is typically good for only about 3,000 cycles and TLC a mere 750 to 1,500 cycles.
3D NAND, which stacks NAND cells one on top of another, can achieve higher storage density without having to shrink the cells as small, which enables higher write endurance. While Samsung has gone back to a 40nm process for its 3D NAND, other flash memory manufacturers such as Micron have decided to use small processes anyway (though not quite as small as planar NAND) to deliver maximum storage density and minimum cost. Typical endurance ratings for 3D TLC NAND are about 2,000 to 3,000 cycles, but can be higher in enterprise-class devices. 3D QLC NAND is typically rated for about 1,000 cycles.
An emerging memory technology called 3D XPoint, developed by Intel and Micron, uses a completely different approach to storing data which is not subject to the endurance limitations of flash memory. 3D XPoint is also vastly faster than flash memory, fast enough to potentially replace DRAM as system memory. Intel will sell devices using 3D XPoint technology under the Optane brand, while Micron will market 3D XPoint devices under the QuantX brand. Consumer SSDs with this technology may hit the market as soon as 2017, although it is my belief that for cost reasons, 3D NAND (primarily of the TLC variety) will be the dominant form of mass storage for the next several years.
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I believe this would be an intresting read for you: http://techreport.com/review/24841/introducing-the-ssd-endurance-experiment
– MustSeeMelons – 2016-08-01T12:07:07.0573
See also https://electronics.stackexchange.com/questions/48395/how-does-memory-wear-out/48399#48399
– pjc50 – 2016-08-01T12:47:07.9174This rests on the precept that there are things you can use forever and never wear down – random – 2016-08-02T13:00:53.593
3http://superuser.com/questions/215463/wear-leveling-at-what-point-will-it-be-unnecessary http://superuser.com/questions/31324/is-flash-drive-wear-a-significant-issue http://superuser.com/questions/410166/how-is-ssd-lifetime-affected-by-usage-pattern-and-amount-of-free-space – random – 2016-08-02T13:35:23.633
1Don't forget the current economy. While physical degradation is a fact. It is most certainly a fact very often defined at the blueprint stage with major factors such as cost and planned obsolescence. – helena4 – 2016-08-03T09:02:14.713
helena4, there is considerable competition to make longer-life flash devices, and it's a difficult enough problem, that I very much doubt that planned obsolescence is any significant factor. However, I'm reasonably sure you're correct that cost is a major factor. There are well-known methods for increasing the write endurance of flash memory, within limits, but it reduces the density so increases the cost per bit. – Eric Smith – 2016-08-04T03:08:17.173
Current flash devices are pretty good (compared to what they used to be like), I haven't seen one fail on a sector-by-sector basis for quite some time, possibly thanks to load-leveling etc.. so the technology is improving. Most flash I've seen die recently has been, presumably, the charge pump https://en.wikipedia.org/wiki/Charge_pump which makes the device stop completely and suddenly! The MTBF of most flash drives are not much worse than rotary HDDs... might be not worth worrying about too much, take sensible precautions and keep a backup routine.
– Michael Stimson – 2016-08-04T05:21:11.067@random "This rests on the precept that there are things you can use forever and never wear down " Doesn't it just rests on the precept that SSDs fail after a much smaller number of writes than the technology that they're replacing? – David Richerby – 2016-08-04T08:20:18.380
The question is "why is there a limited amount of writes?" but doesn't say why there should be unlimited number of writes like some other media. Of which there are none. They are all limited @dav – random – 2016-08-04T12:24:05.387
1@random Only if you insist on interpreting the question in a vacuum and reading "limited" as meaning "having some finite bound" rather than "quite small." For example, if somebody says they "have limited time to contribute to Stack Exchange", I'm pretty sure you'd understand that they mean that they have a small amount of time compared to other users of the site and you wouldn't respond, "I know that: your available time is limited by the fact that you won't live forever." – David Richerby – 2016-08-04T12:37:06.560
@random I clarified what I meant. I did not say that hard drives are usable forever, but especially compared to SSDs, they tend to last a lot longer, disregarding mechanical failure. – Nzall – 2016-08-04T13:11:11.530
@helena4: I'm of the opinion that this isn't planned obsolescence more than it is cost. Drives using TLC NAND are becoming common because they're cheap; minimizing cost per GB trumps endurance because flash is still very expensive compared to mechanical HDD storage and most consumers don't approach the endurance limits for even 15nm planar TLC NAND (even at less than 1000 P/E cycles per block). Remember that endurance is strongly tied to long-term reliability and data loss events aren't good for the manufacturer's bottom line. – bwDraco – 2016-08-06T03:09:15.713
@EricSmith, and bwDraco: You are mistaken on principle and I'll tell you why. The problem with storage life has always been predictability. If the life is limited and you get an OK warning that you need to move before the ship sinks, people will not complain. (and not surprisingly thats what we see being the direction we are moving in). Storage devices are not exempt from common business practices as much as we might wish. – helena4 – 2016-08-08T09:25:37.700