Re: SSDs have extra "unused" space???
On Tue, Jan 21, 2014 at 08:31:56AM -0500, The Wanderer wrote:
> 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.
Picking a random 128 Mbit NAND flash, it has 1024 blocks of 32
pages. A page contains 512 byte user data + 16 byte for things
like ECC and marking it's a bad block. They guarantee that at
least 1004 of those blocks 1024 blocks are usable when the flash
is new. I think they usually also guarantee that at least one of
the first 2 blocks isn't bad. Almost all of the flashes contain
bad blocks when new. And this list will grow as you use it.
Please note that you don't have this problem with NOR flash, but
anything of a reasonable size will be a NAND flash.