Large discs

[Warner Losh]

On Fri, Jan 5, 2018 at 1:44 PM, Paul Koning <paulkoning at comcast.net> wrote:

>
>
> > On Jan 5, 2018, at 3:24 PM, Warner Losh via cctalk <
> cctalk at classiccmp.org> wrote:
> >
> > On Fri, Jan 5, 2018 at 1:13 PM, Fred Cisin via cctalk <
> cctalk at classiccmp.org
> >> wrote:
> >
> >> On Fri, 5 Jan 2018, Mazzini Alessandro wrote:
> >>
> >>> I'm  not sure I would use SSD for long term "secure" storage, unless
> maybe
> >>> using enterprise level ones.
> >>> Consumer level SSD are, by specifics, guaranteed to retain data for 6
> >>> months
> >>>
> >>
> > The JEDEC spec for Consumer grade SSDs is 1 year unpowered at 30C at end
> of
> > life.
> > The JEDEC spec for Enterprise grade SSDs is 90 days, unpowered at 30C at
> > end of life.
>
> That's curious.  Then again, end of life for enterprise SSDs is many
> thousands of write passes over the full disk (or the same amount of writes
> to smaller address ranges thanks to remapping).  Under high but not insane
> loads that takes 5-7 years.  So presumably the retention while fairly new
> (not very worn) is much better.  Still it's surprising to see a number that
> small.
>

There's method in this madness... The enterprise drives are trading
retention for speed and endurance. When you design NAND storage devices,
you basically get to pick two of the following to optimize for: speed,
retention, endurance. Consumer drives are optimized for less endurance,
better retention and decent speed. Enterprise drives are optimized for
maximum speed, good endurance and OK retention.


> > As far as I've seen, all SATA and NVME drive vendors adhere to these
> specs
> > as a minimum, but there's also a new class of drive for 'cold storage'
> > which has high retention, but low endurance and longer data read times...
>
> I don't know if the "cold storage" SSD stuff is going anywhere.  But in
> any case, it seems to aim at high density at the expense of low endurance.
> I don't remember hearing retention discussed at all, higher or unchanged.
>

I've seen briefings that suggest higher retention for that class of
product, but I've not seen the JEDEC profile for it yet.


> Having drives with limited retention seems quite problematic.  And
> "unpowered" suggests that leaving the power on would help -- but I don't
> see why that would be so.
>

That's simple: powered drives have firmware running. The firmware can
manage the NAND such that it reads the data in just enough time to ensure
it can do it reliably before rewriting it to another part of the drive.
Unpowered, it can't do that.


> As for writable DVDs and such, do they have any useful retention specs?
>

No clue there. I worked for 3 years on a NMVE flash drive so got to learn
all about this junk there since my job focused on NAND reliability and
ensuring we met our benchmark numbers for speed, endurance and retention in
the different configuration we sold.

Warner