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Re: file system with no gratuitous writes?

On Jan 3, 2011, at 2:26 AM, Jukka Ruohonen wrote:
> On Sun, Jan 02, 2011 at 04:42:27PM -0500, Steven Bellovin wrote:
>> I'm building a compact flash-based system for a Soekris box.  I'd like to
>> set it up so that there are no write operations that are not specifically
>> initiated by the administrator.
> Slightly off-topic, but I have always wondered how necessary all this
> trouble really is? Has anyone managed to actually kill or wear off a CF
> card due normal operating system disk activity? I have one Soekris running
> on a CF card for over five years now with only /var/log mounted on tmpfs.

I've seen it, but not recently.

Back around 2005 / 2006, we were using a bunch of Soekris 4511's, IIRC, running 
NetBSD and a network IDS we'd been working on, which possibly generated 100s of 
MB to a few GB of logging per day.  Whoever did the initial setup didn't 
realize that the flash cards of that timeframe were limited to 10K writes or 
so, and after a few months you started getting 16K chunks of old logfile data, 
or 16K chunks of new and old logfile data corrupted together-- looked to be a 
binary OR of the 0 bits.

Nothing reported that writes were failing-- evidently the flash cards didn't 
notice an error and thus didn't report it back to the system.  Switching /var 
to tmpfs resolved the issue for us.

From what I understand (a quick review of wikipedia helps :), modern flash 
cards are now typically rated for 100K writes, include ECC bits to actually 
correct or at least detect errors and try to remap bad blocks to unused blocks, 
and implement wear-leveling techniques of varying degrees of effectiveness.


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