SSD NAND decay aka media retention

[valnar]

This has been bugging me for years so I'm finally putting pen to paper to ask the experts.

As NAND gets smaller (50nm, 34nm, 25nm, 20nm, etc) we have been told that write endurance goes down and SSD manufacturers have to employ more tricks to keep the data fresh using wear algorithms, spare space, moving things around, etc. Some sites (like Anandtech) have even gone far enough to write multiple terabytes to select SSD's to see how long they would last. OK, fine.

What I have not seen is any test related to NAND decay, or media retention. What happens if you do NOTHING for X amount of months or years? Wear leveling does not come into play at that point. How long will the data last?

Let's just say, hypothetically, if I want to use an SSD as a USB backup drive and leave it off for months at a time, what kind of problems can I expect with the small 20nm NAND? I suppose the same question can be asked of Flash media too.

Anyone know of any research on that?

 

[glugglug]

Disturbingly, 34nm flash is quoted as having a 5 year retention time, and the newer smaller lithographies even less. Hopefully this figure is very conservative, and the ECC that is there to make the smaller scales practical should also help it.

It isn't really tested for that amount of time, of course -- they make it decay faster with heat and try to calculate a curve for how long it should take at 25C.

 

[piasabird]

For long term storage don't depend on flash. I wonder what happens if you open a file and then save it again? Will that make it last longer? There is no guarantee when it comes to long-term storage. In reality computer files just do not last forever. How long is a guarantee on a Hard Drive? 2 years? 5 Years? DVD's are kind of iffy too. One scratch and say goodbye to your data.

I think I saw some articles about SSD's lasting longer if you use a lot of RAM due to the swapping of disk space.

Maybe if you just plug it in once in a while that might help.

One alternative might be to rent some cloud storage space.

 

[mrpiggy]

SSD's are NOT meant for long term no-power data retention. Current no-power data retention warranted policy for Micron SSD's is 1 year, so I assume most other brands are similar. That is not to say it "won't" retain data longer without power, but that is all Micron will guarantee with its current lineup.

Now if you plug in the drive every so often for a little while, the controllers of the SSD will do a charge check on the SSD's NAND and refresh as necessary, so the data retention period then extends from the time it has been plugged in. I do believe however that as the drive get more usage (hence wearing the NAND more), the physical data retention period capabilities of the NAND gets less as the drive ages,

As for "powered" plugged-in data retention, in a theoretical fantasy land where nothing on an SSD ever goes bad except for the general NAND wear, as long as the controller/PC interface is doing its thing and working properly, there is no reason to fear any general data loss. Of course, in the real world, there is sudden power losses, corrupt SATA transmission, failing SSD controllers, failing MB SATA controllers, failed power regulators, excessive heat, etc, etc that could all cause data corruption/failure. Unfortunately, these types of issues are far more likely to cause SSD data loss or complete failure than any actual NAND data long-term charge failure or wear-out.

The basic premise comes down to, don't trust "any" particular single piece of hardware (SSD, hard drive, DVD, etc) to retain irreplaceable data over long periods of time. Have backups to backups if at all possible.

 

[valnar]

Disturbingly, 34nm flash is quoted as having a 5 year retention time, and the newer smaller lithographies even less. Hopefully this figure is very conservative, and the ECC that is there to make the smaller scales practical should also help it.

It isn't really tested for that amount of time, of course -- they make it decay faster with heat and try to calculate a curve for how long it should take at 25C.

All my SSD's are 34nm at the moment. I just bought an Intel 510 off eBay because it was made on the 34nm process. I think that will help my peace of mind.

It's the specs the manufacturers don't tell that worry me. Of course they are only going to spout the good stuff (speed, etc.).

 

[Hellhammer]

Data retention is dependent on the wear level of the NAND. The more P/E cycles you've used, the shorter the data retention time is (writing/erasing consumes the silicon oxide, which increases the risk that electrons will escape the floating gate).

The official JEDEC spec rules that the data retention time must be one year for consumer devices once the rated P/E cycle count has been achieved (i.e. 3,000 cycles for the current MLC NAND). For a brand new drive the data retention time is obviously much higher, even up to 10 years.

 

[valnar]

Data retention is dependent on the wear level of the NAND. The more P/E cycles you've used, the shorter the data retention time is (writing/erasing consumes the silicon oxide, which increases the risk that electrons will escape the floating gate).

The official JEDEC spec rules that the data retention time must be one year for consumer devices once the rated P/E cycle count has been achieved (i.e. 3,000 cycles for the current MLC NAND). For a brand new drive the data retention time is obviously much higher, even up to 10 years.

I find it hard to believe that the unpowered retention is the same for 50nm SLC as a 20nm TLC process.

 

[Hellhammer]

I find it hard to believe that the unpowered retention is the same for 50nm SLC as a 20nm TLC process.

Of course it's not the same but I haven't seen any data about how it has changed (hence the "up to 10 years" claim). I don't think it really matters because nobody should buy a drive and then throw it in the basement for a decade. Even offsite backups should be checked every now and then to make sure they work properly (e.g. humidity can be harmful in long term) as otherwise you may not have a backup.

 

[Cerb]

I find it hard to believe that the unpowered retention is the same for 50nm SLC as a 20nm TLC process.

The spec is a minimum, and typically at a constant near-room temp. The tech changes so fast that I don't think even the makers actually know how long the regular stuff will last, at least not until it's been out for awhile and they can go back to actual samples. What they can do is be reasonably confident that it will last a year at the rated cycles. Older may last 10 years at 2x its rated cycle count, but they weren't going to even try to offer any assurances of that.

 

[valnar]

How about modern high-capacity flash drives? I know the NAND gets smaller with those 32GB+ USB drives, but I can't find out what.

I'd like to create a self-booting restore USB drive for a computer backup, a laptop I'm setting up for my father. After I've loaded everything on the laptop he cares about, the backup is around 28GB. I figure a 32GB flash drive would hold that plus the OS and backup program to boot it. Obviously any form of optical media would not be big enough.

Question is, how long would it last? I don't even know if USB drives have wear leveling, and in this case it wouldn't matter. I simply need to know how long the media will retain the data. Surely others have asked this question too?

 

[bononos]

How about modern high-capacity flash drives? I know the NAND gets smaller with those 32GB+ USB drives, but I can't find out what.
.......
Question is, how long would it last? I don't even know if USB drives have wear leveling, and in this case it wouldn't matter. I simply need to know how long the media will retain the data. Surely others have asked this question too?

No idea but its definitely less(much much less?) than ssds since they are treated as throwaway devices and to my knowledge no quality standards for usb flash drives unlike ssds.

 

[Fred B]

No idea but its definitely less(much much less?) than ssds since they are treated as throwaway devices and to my knowledge no quality standards for usb flash drives unlike ssds.

Mayby when talking abouth a cheap usb stick , but there are pretty good memory flash cards availible. Compact flash is a good example , these cards are used by high end camera s like some Hasselblads . Sandisk makes a usb memory vault for 100 year storage :|

http://www.sandisk.com/go/preserve/

 

[pcunite]

I want to know the answer to this question too. I need to store data on a 128GB USB key and need to know how long it will be accessible.

 

[Elixer]

I want to know the answer to this question too. I need to store data on a 128GB USB key and need to know how long it will be accessible.

From what I read, pretty much all USB sticks use the 'bottom of the barrel' NAND chips, since SSD's need the higher quality ones.

That means, you need to have multiple backups, on multiple media types to be safe.