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HDDs have awful MTTF compared to SSDs. Where HDDs shine is when you fill them up, unplug them, and put them in a box for decades. SSDs are much more reliable

HDDs have awful MTTF compared to SSDs. Where HDDs shine is when you fill them up, unplug them, and put them in a box for decades. SSDs are much more reliable than HDDs if you want to access the data, they just need to be powered and never written to.

wtallis 5y ago
SSD data retention is also excellent if you only ever fill it up once. Data retention only becomes a problem if you use up most of the write endurance. Whether the SSD is powered or not during the long periods where you're not accessing the data has minimal effect. They're still a horrible data archival solution, but mainly because of the high price per TB.
- dragontamer 5y ago
  > Whether the SSD is powered or not during the long periods where you're not accessing the data has minimal effect. I've heard conflicting reports on this. Ultimately, any storage technology is about storing data. This can be done magnetically (aka: bits are tiny little magnets pointing in certain directions), as long as the magnets don't change, you can read back the data fine. Or it can be done optically (ex: the dye inside of CD-roms, DVD or BluRay). As long as the dye remains a certain color, you can read it back fine. Magnets lose charge over time, dyes can change color (especially if they're exposed to the sun). You won't notice over a year or even 2 years, but after 5, 10, or 15 years, things change enough that you start getting errors. SSDs store data by punching an electrical charge into a transistor. The barrier is such that the electrical charge doesn't want to leak out: but... what is the rate of that leakage? All materials "leak" charge over time. And if the voltage changes dramatically, the change-in-voltage will change the data (and thus you'll lose data over time). -------------------------------- Some SSDs, such as the Samsung 830, were well known for leaking charge and causing issues. Today, we use stacked 3D NAND, which is a different transistor type than the planar NAND on the Samsung 830, but that doesn't fundamentally answer the question of "What is the leakage current" and "how long do we have before enough voltage leaves the transistor to corrupt the data?"
  - wtallis 5y ago
    > I've heard conflicting reports on this. I've seen conflicting reports, too—at least in the popular press and tech forums. Filtering down to reports with clear sourcing back to actual academic or industry research, the confusion pretty much goes away. This is an issue where the actual facts have been oversimplified and dumbed down and wrong interpretations have caught on and been repeated by people with no understanding of the underlying mechanisms for data loss and for forestalling the data loss. In particular, there are tons of people on forums like /r/hardware who think that plugging in the SSD will prevent charge from leaking out of a floating gate memory cell or the charge trap memory cells now used by almost all SSDs. It won't. Using the SSD can lead to data being re-written before it has degraded unrecoverably, through several possible mechanisms. But aside from drives like the Samsung 840 and 840 EVO that were retrofitted with explicit data degradation checks as part of their firmware's operation, merely leaving the drive plugged in will not help directly, and the temperature effects compared to leaving it on a shelf will be a slight detriment to data retention. It's helpful to recall that the JEDEC standards for SSD write endurance require consumer drives to provide 1 year of unpowered data retention for a drive that has reached the end of its write endurance. For brand-new flash, the leakage is orders of magnitude slower than for worn-out flash, so it's quite reasonable to expect data retention comparable to some of the storage mediums that actually make economic sense for archival.
    - Dylan16807 5y ago
      > Using the SSD can lead to data being re-written before it has degraded unrecoverably, through several possible mechanisms. But aside from drives like the Samsung 840 and 840 EVO that were retrofitted with explicit data degradation checks as part of their firmware's operation, merely leaving the drive plugged in will not help directly, and the temperature effects compared to leaving it on a shelf will be a slight detriment to data retention. How about a monthly full drive read? That's easy to set up.
      - wtallis 5y ago
        Yep, a monthly full drive read or even better a scrub with verification of the filesystem's checksums is great. It's not often enough to seriously impact performance in production or meaningfully increase the rate of read disturb errors, but it will allow the SSD to catch data degradation before it becomes a real problem, and to refresh any data that starts showing elevated error rates (or take other measures like start adjusting read voltage thresholds for that block). I don't actually have a cron job set up for it, but I tend to scrub my SSD array every month or two when checking on free space and SMART logs.

wtallis 5y ago

SSD data retention is also excellent if you only ever fill it up once. Data retention only becomes a problem if you use up most of the write endurance. Whether the SSD is powered or not during the long periods where you're not accessing the data has minimal effect. They're still a horrible data archival solution, but mainly because of the high price per TB.

dragontamer 5y ago
> Whether the SSD is powered or not during the long periods where you're not accessing the data has minimal effect. I've heard conflicting reports on this. Ultimately, any storage technology is about storing data. This can be done magnetically (aka: bits are tiny little magnets pointing in certain directions), as long as the magnets don't change, you can read back the data fine. Or it can be done optically (ex: the dye inside of CD-roms, DVD or BluRay). As long as the dye remains a certain color, you can read it back fine. Magnets lose charge over time, dyes can change color (especially if they're exposed to the sun). You won't notice over a year or even 2 years, but after 5, 10, or 15 years, things change enough that you start getting errors. SSDs store data by punching an electrical charge into a transistor. The barrier is such that the electrical charge doesn't want to leak out: but... what is the rate of that leakage? All materials "leak" charge over time. And if the voltage changes dramatically, the change-in-voltage will change the data (and thus you'll lose data over time). -------------------------------- Some SSDs, such as the Samsung 830, were well known for leaking charge and causing issues. Today, we use stacked 3D NAND, which is a different transistor type than the planar NAND on the Samsung 830, but that doesn't fundamentally answer the question of "What is the leakage current" and "how long do we have before enough voltage leaves the transistor to corrupt the data?"
- wtallis 5y ago
  > I've heard conflicting reports on this. I've seen conflicting reports, too—at least in the popular press and tech forums. Filtering down to reports with clear sourcing back to actual academic or industry research, the confusion pretty much goes away. This is an issue where the actual facts have been oversimplified and dumbed down and wrong interpretations have caught on and been repeated by people with no understanding of the underlying mechanisms for data loss and for forestalling the data loss. In particular, there are tons of people on forums like /r/hardware who think that plugging in the SSD will prevent charge from leaking out of a floating gate memory cell or the charge trap memory cells now used by almost all SSDs. It won't. Using the SSD can lead to data being re-written before it has degraded unrecoverably, through several possible mechanisms. But aside from drives like the Samsung 840 and 840 EVO that were retrofitted with explicit data degradation checks as part of their firmware's operation, merely leaving the drive plugged in will not help directly, and the temperature effects compared to leaving it on a shelf will be a slight detriment to data retention. It's helpful to recall that the JEDEC standards for SSD write endurance require consumer drives to provide 1 year of unpowered data retention for a drive that has reached the end of its write endurance. For brand-new flash, the leakage is orders of magnitude slower than for worn-out flash, so it's quite reasonable to expect data retention comparable to some of the storage mediums that actually make economic sense for archival.
  - Dylan16807 5y ago
    > Using the SSD can lead to data being re-written before it has degraded unrecoverably, through several possible mechanisms. But aside from drives like the Samsung 840 and 840 EVO that were retrofitted with explicit data degradation checks as part of their firmware's operation, merely leaving the drive plugged in will not help directly, and the temperature effects compared to leaving it on a shelf will be a slight detriment to data retention. How about a monthly full drive read? That's easy to set up.
    - wtallis 5y ago
      Yep, a monthly full drive read or even better a scrub with verification of the filesystem's checksums is great. It's not often enough to seriously impact performance in production or meaningfully increase the rate of read disturb errors, but it will allow the SSD to catch data degradation before it becomes a real problem, and to refresh any data that starts showing elevated error rates (or take other measures like start adjusting read voltage thresholds for that block). I don't actually have a cron job set up for it, but I tend to scrub my SSD array every month or two when checking on free space and SMART logs.