On Wed, Nov 18, 2020 at 6:00 PM Paul Koning via cctalk <
cctalk@classiccmp.org> wrote:
>
>
> > On Nov 18, 2020, at 5:56 PM, Chuck Guzis via cctalk <
> cctalk@classiccmp.org> wrote:
> >
> > Tangential to this, I've long wondered about some things relating to
> > SSDs. Are there any solid figures on their retention period after years
> > of being unpowered?
> >
> > The reason I ask is that I've long been in the habit of simply shelving
> > an old hard drive when I upgrade or replace a system. I've got hard
> > drives that still work that hail back to the days of OS/2 1.1; some
> > larger ones go back to the 1970s.
>
> You should be able to find the answer in the drive specs.
>
> As I understand it, there are two rather different ranges of answer
> depending on whether you're looking at an enterprise class drive, which is
> optimized for high speed and large total amount of data written, vs. a
> consumer drive. The power-off retention spec is much shorter for the
> enterprise drives. I forgot the numbers; I vaguely remember it being less
> than a year.
>
For SSD devices, based on NAND Flash, the specs for retention are 90 days
for enterprise drives and 1 year for consumer drives, both at 20C. The
difference allows enterprise drives to trade retention for increased write
rate.
> If the drive has power it will do something analogous to DRAM refresh to
> keep the bits in good shape. But it seems that the HDD rule that you can
> just set a drive on the shelf for a decade (ditto with other magnetic
> media) does not necessarily carry over to SSD.
>
Yes. NAND is just a bunch of small capacitors that decay over time. The bit
error rate increases following the arrhenius law. The ECC that goes along
with NAND is paired to allow NAND that's almost worn out to still retain
data for {3 months/1 year} given its expected bit error rate when it's
almost worn out when programmed, coupled with the expected decay during the
specified retention time.
Also note I said "at 20C." The acceleration effect can be quite pronounced
should the data center suffer some catastrophic event that leaves it
without power in a super hot environment for weeks or months. At ~70C the
acceleration factor can be as high as 30-90x, which can render enterprise
drives not reliable after a few days baking at high temperatures.
Brand new NAND, on the other hand, typically has retention capabilities
measured in years or tens of years. It's the wear and tear of use that
makes it less reliable, often much less reliable. And the multi level per
cell technologies are much worse than the single level per cell. It's one
reason that the smaller number of bits per cell NAND tends to last longer
than larger bits per cell, all other things being equal. The smaller
process sizes also were less reliable since they could store fewer
electrons (sometimes as few as a dozen or two per state). 3D NAND was so
much better because it could grow vertically, allowing NAND manufacturers
to return to larger process sizes and still increase density, also giving
better endurance for a time...
Warner
