Lars Noodén via Gnupg-users wrote:
> On 5/5/22 01:11, Jacob Bachmeyer wrote:
> > Lars Noodén via Gnupg-users wrote:
> >> A removable hard drive might be an option, if the storage time
> >> is less than a decade and there are decent storage conditions
> >> in regards to chemicals, temperature, humidity, and so on. Flash
> >> memory seems to lose
> >> its charge rather quickly, measured in months.
> >
> > Write-once optical media is my preferred means of long-term backup for
> > nontrivial amounts of data,
> [snip]
>
> The number of years that the keys and the data they apply to will be
> stored unpowered, offline will influence which storage medium is
> acceptable for the task.
>
> Old CD-R were short-lived garage from my experience, but certain models
> of recently made CD-R should last a while even under slightly
> non-optimal storage conditions before they start flipping bits.
This depends on the quality of the media. I first got a CD-R drive in
the mid 2000s and have discs from back then that were still readable
when I last looked at them a few years ago. Admittedly, these have been
stored under ordinary room conditions and protected in a disc binder or
jewel cases and were not the "bargain basement" media that was also
available at the time. A friend once lamented having something like 3
to 5 discs out of a 100-pack of "Great Quality" branded CD-R media that
were actually usable; the rest were either rejected during burning or
failed immediately upon readback. It is doubtful that those "Great
Quality" discs are still readable today. There was a significant
difference in price: the discs I used (Maxell/Memorex/Verbatim name
brands stand out thinking back) typically cost about $20 for a 50-pack
or similar for a 100-pack if on sale, while "Great Quality" was $5 for
100. You really did get what you paid for, however.
There were also direct-write DVD-R camcorders fairly popular in the mid
to late 2000s. I remember news stories about most of Barack Obama's
earlier speeches having been lost before his first term as US President
had ended because the only recordings had been made by his supporters
using those camcorders and cheap DVD-R media that did not last.
Note that "nontrivial amounts of data" excludes PGP keys; even a
mini-CD-R holds several megabytes. I will admit that lack of a
reasonable backup strategy is one of the reasons I do not presently use
PGP for encryption.
> [...]
>
> Whether that bit flip hits anything important is another matter, but
> they do add up over time and with enough of them they will eventually
> hit something, worse if it hit something compressed. [...]
CD-ROM format has considerable data expansion. If I remember correctly,
a 650MB data CD actually stores something like 2.1GB after applying the
various ECC layers. There are quite a few bits to flip before anything
is affected.
> Air pollution, temperature, light, and humidity are some of the factors
> affecting the lifespan of the physical storage medium.
One of the advantages of optical media generally is that the discs are
(supposed to be) sealed against their environment. Absent extremes,
(polycarbonate has a melting point, the data is written using very
intense light that locally heats the dye layer) environmental effects
should be minimal. Along these lines, while fire will obviously destroy
optical media, discs should remain readable after being in a flood, for
example. (Some mold removal may be needed, and the data should be
copied to new media in case mold or the chemicals used to remove it
adversely affect the integrity of the environmental seal.)
> > I have SD cards and USB sticks with data blocks last written
> > many years ago and still readable. Granted, I have never used
> > low-end no-name
> [snip]
>
> And by reading them, they have powered up and refreshed the charge. The
> problem applies to such flash storage devices which have been left
> unpowered for longer periods of time.
No, it does not. That is not how flash memory works. Some flash
translation layers might do such things in some devices, but I strongly
doubt that flash-based microcontrollers have undocumented hardware
functions to periodically rewrite the program storage, for example. In
any case, I have both USB sticks and SD cards that have been left
entirely unpowered for years and found the data to still be there,
certainly much longer than the "few months" you mentioned earlier.
Theoretically, the stored charge does eventually leak off of the
floating gate, but EEPROMs (and flash, which is essentially the same
technology) are generally considered to hold data indefinitely. The
data retention specifications are based on "accelerated aging" tests,
which generally involve elevated temperature. The processes involved
are highly nonlinear with respect to temperature and may very easily
require centuries at room temperature or not occur at all without
elevated temperatures; we do not know because flash storage is only now
reaching the milestone of having existed long enough for even the oldest
imprints to be reaching even the "accelerated aging" estimated
lifespan. So far, we are not seeing catastrophic losses of data stored
in flash.
-- Jacob
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