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Unpowered SSDs slowly lose data

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Re: Unpowered SSDs slowly lose data

#161

Post by userbinator »

bullen wrote: post #153
karczex wrote: post #142

Earlier quoted context omitted.

That's how it has to work. To increase capacity you have to make smaller cells where charge may easier diffuse from one cell to another. Also to make drive faster, stored charge has to be smaller, which also decrease endurance. With SLC and QLC comparison is even worse as QLC is basically clever hack to store 4 times more data in the same number physical cells - it's tradeoff.

Yes, but that tradeoff comes with a hidden cost: complexity! I much rather have 64GB of SLC at 100K WpB than 4TB of MLC at less than 10K WpB. The spread functions that move bits around to even the writes or caches will also fail. The best compromise is of course to use both kinds for different purposes: SLC for small main OS (that will inevitably have logs and other writes) and MLC for slowly changing large data like…

The problem is now you cannot choose because the factories/machines that make SLC are all gone.

You can still get pure SLC flash in smaller sizes, or use TLC/QLC in SLC mode.

I much rather have 64GB of SLC at 100K WpB than 4TB of MLC at less than 10K WpB.

It's more like 1TB of SLC vs. 3TB of TLC or 4TB of QLC. All three take the same die area, but the SLC will last a few orders of magnitude longer.

Re: Unpowered SSDs slowly lose data

#162

Post by ACCount37 »

userbinator wrote: post #77

One key point about retention which is not often mentioned, and indeed neither does this article, is that retention is inversely proportional to program/erase cycles and decreases exponentially with increasing temperature. Hence why retention specs are usually X amount of time after Y cycles at Z temperature. Even a QLC SSD that has only been written to once, and kept in a freezer at -40, may hold data for several de…

Because no one is willing to pay for SLC.

Those QLC NAND chips? Pretty much all of them have an "SLC mode", which treats each cell as 1 bit, and increases both write speeds and reliability massively. But who wants to have 4 times less capacity for the same price?

Re: Unpowered SSDs slowly lose data

#163

Post by pbmonster »

dachris wrote: post #151
pbmonster wrote: post #133

Earlier quoted context omitted.

I know we're talking theoretical optimums here, but: don't put your SSDs in the freezer. Water ingress because of condensation will kill your data much quicker than NAND bit rot at room temperature.

Would an airtight container and liberal addition of dessicants help?

Sure. Just make sure the drive is warm before you take it out of the container - because this is when the critical condensation happens: you take out a cold drive an expose it to humid room temperature air. Then water condenses on (and in) the cold drive.

Re-freezing is also critical, the container should contain no humid air when it goes into the freezer, because the water will condense and freeze as the container cools. A tightly wrapped bag, desiccant and/or purging the container with dry gas would prevent that.

Re: Unpowered SSDs slowly lose data

#165

Post by ACCount37 »

bullen wrote: post #160
fairfeather wrote: post #54

Earlier quoted context omitted.

SSD firmware engineer here. I work on enterprise stuff, so ymmv on consumer grade internals. Generally, the data refresh will all happen in the background when the system is powered (depending on the power state). Performance is probably throttled during those operations, so you just see a slightly slower copy while this is happening behind the scenes. The unused space decaying is probably not an issue, since the int…

Ok, so all bits have to be rotated, even when powered on, to not loose their state? Edit: found this below: "Powering the SSD on isn't enough. You need to read every bit occasionally in order to recharge the cell." Hm, so does the firmware have a "read bits to refersh them" logic?

Kind of. It's "read and write back" logic, and also "relocate from a flaky block to a less flaky block" logic, and a whole bunch of other things.

NAND flash is freakishly unreliable, and it's up to the controller to keep this fact concealed from the rest of the system.

Re: Unpowered SSDs slowly lose data

#166

Post by consp »

londons_explore wrote: post #155
userbinator wrote: post #77

One key point about retention which is not often mentioned, and indeed neither does this article, is that retention is inversely proportional to program/erase cycles and decreases exponentially with increasing temperature. Hence why retention specs are usually X amount of time after Y cycles at Z temperature. Even a QLC SSD that has only been written to once, and kept in a freezer at -40, may hold data for several de…

I'm sad that drives don't have a 'shutdown' command which writes a few extra bytes of ECC data per page into otherwise empty flash cells. It turns out that a few extra bytes can turn a 1 year endurance into a 100 year endurance.

Blind question with no attempt to look it up: why don't filesystems do this? It won't work for most boot code but that is relatively easy to fix by plugging it in somewhere else.

Re: Unpowered SSDs slowly lose data

#167

Post by londons_explore »

userbinator wrote: post #147
lotrjohn wrote: post #108

Earlier quoted context omitted.

Completely anecdotal, and mostly unrelated, but my NES from 1990 is still going strong. Two PS3’s that I have owned simply broke. CRTs from 1994 and 2002 still going strong. LCD tvs from 2012 and 2022 just went kaput for no reason. Old hardware rocks.

LCD tvs from 2012 and 2022 just went kaput for no reason. Most likely bad capacitors. The https://en.wikipedia.org/wiki/Capacitor_plague may have passed, but electrolytic capacitors are still the major life-limiting component in electronics.

MLCC's look ready to take over nearly all uses of electrolytics.

They still degrade with time, but in a very predictable way.

That makes it possible to build a version of your design with all capacitors '50 year aged' and check it still works.

Sadly no engineering firm I know does this, despite it being very cheap and easy to do.

Re: Unpowered SSDs slowly lose data

#168

Post by userbinator »

ACCount37 wrote: post #162
userbinator wrote: post #77

One key point about retention which is not often mentioned, and indeed neither does this article, is that retention is inversely proportional to program/erase cycles and decreases exponentially with increasing temperature. Hence why retention specs are usually X amount of time after Y cycles at Z temperature. Even a QLC SSD that has only been written to once, and kept in a freezer at -40, may hold data for several de…

Because no one is willing to pay for SLC. Those QLC NAND chips? Pretty much all of them have an "SLC mode", which treats each cell as 1 bit, and increases both write speeds and reliability massively. But who wants to have 4 times less capacity for the same price?

4 times less capacity but 100x or more endurance or retention at the same price looks like a great deal to me. Alternatively: do you want to have 4x more capacity at 1/100th the reliability?

Plenty of people would be willing to pay for SLC mode. There is an unofficial firmware hack that enables it: https://news.ycombinator.com/item?id=40405578

1TB QLC SSDs are But why won't the manufacturers let you choose? The real answer is clearly planned obsolescence.

I have an old SLC USB drive which is only 512MB, but it's nearly 20 years old and some of the very first files I wrote to it are still intact (I last checked several months ago, and don't expect it's changed since then.) It has probably had a few hundred full-drive-writes over the years --- well worn-out by modern QLC/TLC standards, but barely-broken-in for SLC.

Re: Unpowered SSDs slowly lose data

#169

Post by medoc »

homebrewer wrote: post #158
geokon wrote: post #152

Earlier quoted context omitted.

noob question... how do i force a full read?

the most basic solution that will work for every filesystem and every type of block device without even mounting anything, but won't actually check much except device-level checksums: sudo pv -X /dev/sda or even just: sudo cat /dev/sda >/dev/null and it's pretty inefficient if the device doesn't actually have much data, because it also reads (and discards) empty space. for copy-on-write filesystems that store checksu…

Just as a note, and I checked that it's not the case with the GNU coreutils: on some systems, cp (and maybe cat) would mmap() the source file. When the output is the devnull driver, no read occurs because of course its write function does nothing... So, using a pipe (or dd) maybe a good idea in all cases (I did not check the current BSDs).

Re: Unpowered SSDs slowly lose data

#170

Post by big-and-small »

ACCount37 wrote: post #162
userbinator wrote: post #77

One key point about retention which is not often mentioned, and indeed neither does this article, is that retention is inversely proportional to program/erase cycles and decreases exponentially with increasing temperature. Hence why retention specs are usually X amount of time after Y cycles at Z temperature. Even a QLC SSD that has only been written to once, and kept in a freezer at -40, may hold data for several de…

Because no one is willing to pay for SLC. Those QLC NAND chips? Pretty much all of them have an "SLC mode", which treats each cell as 1 bit, and increases both write speeds and reliability massively. But who wants to have 4 times less capacity for the same price?

To be honest you can buy 4TB SSD for $200 now, so I guess market would be larger if people were aware of how easy would it be to make such SSDs work in SLC mode exclusively.
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