[nzlug] machine pre loaded with linux

[Robert Coup]

Vik Olliver wrote:
> On Thu, 2007-02-08 at 10:00 +1300, Robert Coup wrote:
>   
>> An onboard SD connector is a few cents, it could connect as a USB
>> device if the designer has no engineering skills, and suddenly you
>> have 512MB of swap for <US$10.
>>     
>
> Flash memory has a write limit of 100,000 cycles. Your SD card is now a
> consumable...
>   
I call FUD.

With proper wear balancing (and a proper flash disk) your flash disk 
should last easily for several million writes - they're normally 
/guaranteed/ for at least 1 million write/erase cycles 
(http://en.wikipedia.org/wiki/Flash_memory) and from my previous job we 
got guarantees of 3-5m cycles from manufacturers (no source). Wear 
balancing and error correction is normally handled by the card 
controller. However the filesystem can do it for "dumb" controllers 
(JFFS/JFFS2 are examples).

ok, so lets do some maths: How many years the flash disk lasts writing W 
blocks/second before you actually lose a flash block:

Assumptions:
- errors/remapping are handled by the card controller up until it hits 
the below point
- wear levelling happens
- the flash can handle 1 million write/erase cycles
- flash block size is 512bytes

C = capacity (MB), W = written 64KB blocks (typical filesystem size) per 
second.

(C*10^6 bytes / 512 * 10^6 write cycles)
/
((W*64*1024/512 flash blocks) * 60 * 60 * 24 * 365.25 block writes per year)

For a 512MB flash drive this should be 247 years.
For a 1GB flash drive this should be 495 years.

Rob :)