[Soekris] CF devices as swap?

[Joel Jaeggli]

Malcolm Herbert wrote:
> I've recently re-discovered the CF disk that came with my net4801 and
> was wondering what use it would be as a swap device ... I'm aware of
> the fact that CF media doesn't last indefinately, but I'd be interested
> in hearing whether others had tried this and how long it lasted before
> giving problems ... 

Assuming your wear leaveling algrithm works... with a very fast 4GB 
flash card.

30MB/s * 86400 = 2.592TB per day = 648 cycles

100,000 cycle rated flash / 648 cycles per day = 154 days of continious 
writing.

Most flash components are slower than that. and of course the bigger 
they are the more area they have to use for wear leveling.

> unfortunately swap problems are generally fatal, so I may just decide
> to stick with swap on the attached HDD ... 

flash memory controllers generally implement ecc sufficient to repair 
multibit errors. it in not a panace and at the point where you start 
lossing whole blocks you have a problem (smart can tell you a bit about 
the flash if the controller implments it) but the products are becoming 
quite mature. I prefer not to built embedded systems that swap to flash 
but I'll do it in a pinch.

If you're building a bunch of embedded systems, find an industrial flash 
manufacturer the makes their own controller firmware and have them 
validate their product for your workload.