3 ms·

I think they crossed the limit of sacrifice that is understandable given the endurance of the materials currently used in NAND. Cell modes are done by charging

by drtgh 3y ago

I think they crossed the limit of sacrifice that is understandable given the endurance of the materials currently used in NAND. Cell modes are done by charging a cell to a level on write, and measuring the voltage on read. The more bits per cell, the more voltage divisions, and the sooner these voltages divisions turns unreadable, while the material degrades. Technically is on the manufacturer's firmware to decide what cell mode is going to use the NAND, assuming the hardware allows it (ADC/DAC, etc). But even if the data is not changing very often, the controller of the disk needs to do internal writes for to refresh the cell, or even change the data of cell, in order to avoid data loss. So IMHO, they are not giving more capacity, due that capacity is going to be consumed for to try to keep alive the data. If one fills the QLC disk of the article for long term storage, bad things will happen, as I understand it. May be should be the user who decide what bit mode must use the disk. But what is happening, I think, is along years they are stretching like chewing gum the NAND incrementing the bits by cell, hopping the people adopt the reduced endurance as a normality at prices of data-stored, not of NAND chips-used, at time they limit their chips production.

wtallis 3y ago
> So IMHO, they are not giving more capacity, due that capacity is going to be consumed for to try to keep alive the data. If one fills the QLC disk of the article for long term storage, bad things will happen, as I understand it. Your opinion aside, QLC drives really do offer more capacity for your dollar, and the extra write cycles necessary to stave off data degradation do not render the drives useless. You absolutely can fill a QLC drive and leave the data on there. You may need to perform scrubs a bit more often than you would for an equivalent TLC drive if you're not writing enough new data to keep things fresh, but that's hardly a showstopper. > But what is happening, I think, is along years they are stretching like chewing gum the NAND incrementing the bits by cell, hopping the people adopt the reduced endurance as a normality at prices of data-stored, not of NAND chips-used People have been predicting a catastrophe of low write endurance for many years, and it keeps not happening. There's some plausibility to the notion that consumers may end up getting screwed over by cheap drives that cannot hold up under reasonable usage, but consumer QLC drives have now been on the market for five years: the time limit on those drive warranties is now expiring, with no sign of an epidemic of premature drive failures. And in the enterprise SSD space, it is foolhardy to presume the big customers that would consider buying 256TB SSDs do not understand their workloads and endurance requirements well enough to correctly determine whether QLC is suitable for their needs.
- drtgh 3y ago
  > but consumer QLC drives have now been on the market for five years: the time limit on those drive warranties is now expiring, with no sign of an epidemic of premature drive failures The failure is not shown as an epidemic, it is shown firstly as a degradation; in forums under questions about why turned so slow the disk that find as answer to enable the RAPID mode (so using RAM cache), in other post as regrets about the purchase, and things like that. those who know what kind of thing they are buying with QLC store disposable data, at time they do not fill the disk.
  - wtallis 3y ago
    I really don't see how conflating performance problems with data retention problems helps illustrate your point.
    - drtgh 3y ago
      Firstly arise the performance problem, if one keeps using it comes the data progressive loss, those are the ones at forums talking about high trim data errors.
      - deleted 3y ago
        [deleted]
      - drtgh 3y ago
        [2018] doi.org/10.1145/3224432 https://people.inf.ethz.ch/omutlu/pub/3D-NAND-flash-lifetime-early-retention-loss-and-process-variation_sigmetrics18_pomacs18-twocolumn.pdf https://people.inf.ethz.ch/omutlu/pub/3D-NAND-flash-lifetime... [2021] doi.org/10.1145/3445814.3446733 (use sci-hub) " 3D NAND density-increasing techniques, such as extensive stacking of cell layers, can amplify read disturbances and shorten SSD lifetime. From our lifetime-impact characterization on 8 state-of-the-art SSDs, we observe that the 3D TLC/QLC SSDs can be worn-out by low read-only workloads within their warranty period since a huge amount of read disturbance-induced rewrites are performed in the background. " [..] " the SSDs entered an era where one can wear out an SSD by simply reading it. "

by drtgh 3y ago

wtallis 3y ago

> So IMHO, they are not giving more capacity, due that capacity is going to be consumed for to try to keep alive the data. If one fills the QLC disk of the article for long term storage, bad things will happen, as I understand it. Your opinion aside, QLC drives really do offer more capacity for your dollar, and the extra write cycles necessary to stave off data degradation do not render the drives useless. You absolutely can fill a QLC drive and leave the data on there. You may need to perform scrubs a bit more often than you would for an equivalent TLC drive if you're not writing enough new data to keep things fresh, but that's hardly a showstopper. > But what is happening, I think, is along years they are stretching like chewing gum the NAND incrementing the bits by cell, hopping the people adopt the reduced endurance as a normality at prices of data-stored, not of NAND chips-used People have been predicting a catastrophe of low write endurance for many years, and it keeps not happening. There's some plausibility to the notion that consumers may end up getting screwed over by cheap drives that cannot hold up under reasonable usage, but consumer QLC drives have now been on the market for five years: the time limit on those drive warranties is now expiring, with no sign of an epidemic of premature drive failures. And in the enterprise SSD space, it is foolhardy to presume the big customers that would consider buying 256TB SSDs do not understand their workloads and endurance requirements well enough to correctly determine whether QLC is suitable for their needs.

drtgh 3y ago
> but consumer QLC drives have now been on the market for five years: the time limit on those drive warranties is now expiring, with no sign of an epidemic of premature drive failures The failure is not shown as an epidemic, it is shown firstly as a degradation; in forums under questions about why turned so slow the disk that find as answer to enable the RAPID mode (so using RAM cache), in other post as regrets about the purchase, and things like that. those who know what kind of thing they are buying with QLC store disposable data, at time they do not fill the disk.
- wtallis 3y ago
  I really don't see how conflating performance problems with data retention problems helps illustrate your point.
  - drtgh 3y ago
    Firstly arise the performance problem, if one keeps using it comes the data progressive loss, those are the ones at forums talking about high trim data errors.
    - deleted 3y ago
      [deleted]
    - drtgh 3y ago
      [2018] doi.org/10.1145/3224432 https://people.inf.ethz.ch/omutlu/pub/3D-NAND-flash-lifetime-early-retention-loss-and-process-variation_sigmetrics18_pomacs18-twocolumn.pdf https://people.inf.ethz.ch/omutlu/pub/3D-NAND-flash-lifetime... [2021] doi.org/10.1145/3445814.3446733 (use sci-hub) " 3D NAND density-increasing techniques, such as extensive stacking of cell layers, can amplify read disturbances and shorten SSD lifetime. From our lifetime-impact characterization on 8 state-of-the-art SSDs, we observe that the 3D TLC/QLC SSDs can be worn-out by low read-only workloads within their warranty period since a huge amount of read disturbance-induced rewrites are performed in the background. " [..] " the SSDs entered an era where one can wear out an SSD by simply reading it. "