The best hard drives??

Bill Gunshannon bill.gunshannon at
Wed Nov 18 19:23:45 CST 2020

On 11/18/20 8:16 PM, Warner Losh via cctalk wrote:
> On Wed, Nov 18, 2020 at 6:00 PM Paul Koning via cctalk <
> cctalk at> wrote:
>>> On Nov 18, 2020, at 5:56 PM, Chuck Guzis via cctalk <
>> cctalk at> 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...

An interesting write-up. Brings up a question on a slightly related
item.  Do Compact Flash and SD have the same short life when not
powered?  What things like Flash Memory used to hold firmware on
other kinds of chips.


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