Re: SSDs have extra "unused" space???
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On 01/20/2014 09:34 PM, Paul Wise wrote:
> On Tue, Jan 21, 2014 at 1:22 AM, Holger Levsen wrote:
>
>> wait, what? Do you have any vendor statements to support this 20%
>> extra space?
>
> Flash is basically probabilistic storage and you need extra space to
> ensure that the probability your data is stored remains high,
That doesn't match my understanding, and I don't see anything to support
it in the reference links you gave.
My understanding is that flash is unreliable in that individual flash
cells will die (cease to retain what is written) after "too many"
write/erase cycles, where the value of "too many" goes down as the
fabrication process gets smaller. To mitigate the real-world effect of
this and extend drive life, manufacturers build in more flash than the
reported drive capacity, a practice known as "overprovisioning".
Modifying the drive firmware can make it possible to access this flash
as normal storage, at the cost of drive lifetime.
The reference links you provided do state that flash memory is
"inherently unreliable and unpredictably fragmented", and one of them
also states that a large fraction of the memory on any given flash die
is likely to be bad sectors, but nothing I've noticed in them supports
the claim that there's anything probabilistic about it.
I do find one research-paper PDF, by a Google search, which describes
techniques for allowing writes to flash memory at lower voltages than
otherwise required; one of those techniques involves writing the data to
multiple places in the hopes that at least one place will take the
write, but there is no indication that this technique is widely enough
used to justify the generality of the statement, and in any case that
would be an implementation detail rather than a basic fact of flash
memory.
Is there something I've missed?
- --
The Wanderer
Secrecy is the beginning of tyranny.
A government exists to serve its citizens, not to control them.
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