On Mon, Feb 9, 2009 at 10:14 AM, Jim Jackson <> wrote:
>
> That's good. Re-using just a small percentage of a large flash card, allows
> the wear leveling to cycle thru' a higher percentage of free blocks, and so
> means the flash lasts longer.
>
> One of the worse situations is to have a card 95% full with static data,
> and you are rewriting data in the remaining 5%. The wear levelling can only
> cycle round the small percentage of free blocks, so they wear out quicker.
The wear-leveling behavior you described here is known as "dynamic
wear-leveling." This means the empty or unused flash blocks and the
blocks that are dynamic, or changing, in the current transaction are
the only ones involved with the wear-leveling by the controller. Some
controllers also include any spare reserve blocks in the dynamic
wear-leveling.
As of a few years ago, a better algorithm has been deployed by most
flash controller makers. This is known as "static wear-leveling" and
includes ALL flash blocks, including those with static data. In
static wear-leveling the writes are moved all around the flash media,
even moving around previously written blocks of user data. This
obviously improves the spread out of the wear on the flash.
Which does your controller use? Well, that's hard to know for all the
reasons we have already discussed in this thread. However, over the
past two years or so all controllers have been moving to the static
wear-leveling model to get maximum life out of the flash. If you
bought your SD cards within the last two years or so, you should be
enjoying the better algorithm. Also, if your memory card is billed as
"industrial" or "extended wear" or something like that, it almost
positively has the better algorithm or static wear-leveling in use.
Alan
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