future: SSD as backup archive ?

Started Dec 24, 2008 | Discussions thread
Expert Photo Recovery
Regular MemberPosts: 170
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Manufacturers do quote a limited flash memory retention period
In reply to toronto123, Dec 30, 2008

Hi,

toronto123 wrote:

john wrote:

no matter you use it or leave it there, the memory will only last
within 10 years, you can see the statement in each CF package.

John ... this is the first time I have ever heard this. And I very,
very seriously doubt it. Do you have any solid sources of this claim ?

Everywhere one looks for info on this subject one will find mention
of the limited number of "write cycles". But your comment of "within
ten years", I have never seen that before.

I think John was referring to what is called "retention" i.e. how long would the data still be readable, if the NAND flash memory was written to, and then left in a cupboard.

And yes, 10 years is in the upper range of what I see manufacturers currently quoting for consumer NAND flash retention - I've seen "up to 5 years" quoted sometimes as well. In general, MLC NAND tends to have shorter retention than SLC, due to the inherent smaller differences between the internal voltages on the floating gates, representing the different logic levels. It's the (tiny!) charge leaking from the floating gates over time, which limits retention.

Here's Kingston quoting "up to 10 years" retention:

http://www.kingston.com/products/pdf_files/FlashMemGuide.pdf

"Kingston Flash storage devices are rated for up to 10 years under normal use. Important information should also be backed up on other media for long-term safekeeping."

There's also a link between retention and endurance - flash memory cells which have been written to frequently, have a shorter retention period. This in-depth presentation from memory manufacturer Micron, confirms this (see pages 21-23):

http://download.micron.com/pdf/presentations/events/flash_mem_summit_jcooke_inconvenient_truths_nand.pdf

However, as that presentation explains, you can erase & re-program the flash memory cells, and the retention "clock" starts again (just try to minimise that "refreshing", to avoid causing endurance-related effects). So it's not that the NAND flash actually fails after that "up to" 5-10 years, but the data may not be able to be reliably read, without being refreshed. Also I haven't seen a minimum guaranteed retention period specified when looking at normal NAND flash datasheets, so some people may have problems more quickly than 5-10 years.

So for this reason, I don't personally consider NAND flash to currently be suitable for "save-and-forget" archiving; but neither are hard disk drives - blackhawk13's experience here with hard disks becoming unreadable just sitting on a shelf, is not unusual:

http://forums.dpreview.com/forums/readflat.asp?forum=1023&message=30461684

Until you actually try to read each block on a hard disk (or NAND flash, or any other storage) you don't know whether it is still readable, or not. That's why some form of archival management, including the regular reading and refreshing (or migrating) of data, unfortunately needs to be considered by people who are serious about digital archiving.

Hope that helps,
--
Sam

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