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Unpowered SSDs slowly lose data

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Re: Unpowered SSDs slowly lose data

#171

Post by formerly_proven »

Yokolos wrote: post #17
formerly_proven wrote: post #8

> Even the cheapest SSDs, say those with QLC NAND, can safely store data for about a year of being completely unpowered. More expensive TLC NAND can retain data for up to 3 years, while MLC and SLC NAND are good for 5 years and 10 years of unpowered storage, respectively. This is somewhat confused writing. Consumer SSDs usually do not have a data retention spec, even in this very detailed Micron datasheet you won't f…

Any given TBW/DWPD values are irrelevant for unpowered data retention. Afaik, nobody gives these values in their datasheet and I'm wondering where their numbers are from, because I've never seen anything official. At this point I'd need to be convinced that the manufacturers even know themselves internally, because it's never been mentioned by them and it seems to be outside the intended use cases for SSDs

Example: https://www.ssd.group/wp-content/uploads/2022/07/PM1733-25-S...

> Component Design Life 5 years

> TBW 14 PB for 7.68 TB drives

> Data Retention 3 months

And then 2.7.3 explains that this number applies for 40 °C ambient, not the operating/non-operating range (up to 85 °C).

Re: Unpowered SSDs slowly lose data

#172

Post by big-and-small »

userbinator wrote: post #168
ACCount37 wrote: post #162

Earlier quoted context omitted.

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…

Funny enough I just managed to find this exact post and comment on google 5 minutes ago when I started wondering whatever it's actually possible to use 1/4 of capacity in SLC mode.

Though what make me wonder is that some reviews of modern SSDs certainly mention that that pSCL is somewhat less than 25% of capacity, like 400GB pSLC cache for 2TB SSD:

https://www.tomshardware.com/pc-components/ssds/crucial-p310...

So you get more like 20% of SLC capacity at least on some SSDs

Re: Unpowered SSDs slowly lose data

#173

Post by londons_explore »

rossjudson wrote: post #124
dale_glass wrote: post #46

So on the off-chance that there's a firmware engineer in here, how does this actually work? Like does a SSD do some sort of refresh on power-on, or every N hours, or you have to access the specific block, or...? What if you interrupt the process, eg, having a NVMe in an external case that you just plug once a month for a few minutes to just use it as a huge flash drive, is that a problem? What about the unused space,…

Keep in mind that when flash memory is read, you don't get back 0 or 1. You get back (roughly) a floating point value -- so you might get back 0.1, or 0.8. There's extensive code in SSD controllers to reassemble/error correct/compensate for that, and LDPC-ish encoding schemes. Modern controllers have a good idea how healthy the flash is. They will move data around to compensate for weakness. They're doing far more to…

Unless I am misunderstanding the communication protocol between the flash chip and the controller, there is no way for the controller to know that analogue value. It can only see the digital result.

Maybe as a debug feature some registers can be set up adjust the threshold up and down and the same data reread many times to get an idea of how close certain bits are to flipping, but it certainly isn't normal practice for every read.

Re: Unpowered SSDs slowly lose data

#174

Post by londons_explore »

breput wrote: post #157

The spinrite[0] user group has noticed some of these effects, even on in-service drives. The theory is that operating system files, which rarely change, are written and almost never re-written. So the charges begin to decay over time and while they might not be unreadable, reads for these blocks require additional error correction, which reduces performance. There have been a significant number of (anecdotal) reports…

There are lots of other potential causes for the same effect...

Eg. Data structures used for page mapping getting fragmented and therefore to access a single page written a long time ago requires checking hundreds of versions of mapping tables.

Re: Unpowered SSDs slowly lose data

#175

Post by victorbjorklund »

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.

That does sound like a good idea (even if I’m sure some very smart people know why it would be a bad idea)

Re: Unpowered SSDs slowly lose data

#176

Post by userbinator »

jcynix wrote: post #57

Hmm, so what about these modern high density hard drives which store track parameters for their servos in on-board flash (aka OptiNAND)? Do we get "spinning rust" which might loose the information where exactly it stored the data? https://blog.westerndigital.com/optinand-explained/

I think you're talking about two different things; "adaptives" are usually stored in EEPROM or the MCU's NOR flash, which is basically better-than-SLC levels of reliability, especially as they're written once and treated as ROM after that.

OptiNAND is a "SSHD" and thus has the same concerns with retention as an SSD. https://en.wikipedia.org/wiki/Hybrid_drive

Re: Unpowered SSDs slowly lose data

#177

Post by [deleted] »

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?

[deleted]

Re: Unpowered SSDs slowly lose data

#178

Post by userbinator »

zozbot234 wrote: post #50
BaardFigur wrote: post #39

I don't use my drive much. I still boot it up snd write some data, just not the long term one. Am I in risk?

AIUI, informal tests have demonstrated quite a bit of data corruption in Flash drives that are literally so worn out that they might as well be about to fail altogether - well beyond any manufacturer's actual TBW specs - but not otherwise, least of all in new drives that are only written once over for the test. It seems that if you don't wear out your drive all that much you'll have far less to worry about.

No, even drives written once have started showing a rise in (correctable) errors after 2 years: https://news.ycombinator.com/item?id=43739028

Re: Unpowered SSDs slowly lose data

#179

Post by londons_explore »

consp wrote: post #166
londons_explore wrote: post #155

Earlier quoted context omitted.

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.

The filesystem doesn't have access to the right existing ECC data to be able to add a few bytes to do the job. It would need to store a whole extra copy.

There are potentially ways a filesystem could use heirarchical ECC to just store a small percentage extra, but it would be far from theoretically optimal and rely on the fact just a few logical blocks of the drive become unreadable, and those logical blocks aren't correlated in write time (which I imagine isn't true for most ssd firmware).

Re: Unpowered SSDs slowly lose data

#180

Post by DeepSeaTortoise »

consp wrote: post #166
londons_explore wrote: post #155

Earlier quoted context omitted.

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.

You can still do this for boot code if the error isn't significant enough to make all of the boot fail. The "fixing it by plugging it in somewhere else" could then also be simple enough to the point of being fully automated.

ZFS has "copies=2", but iirc there are no filesystems with support for single disk erasure codes, which is a huge shame because these can be several orders of magnitude more robust compared to a simple copy for the same space.

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