Re: [gentoo-embedded] file system question

[Ed W]

On 29/03/2010 23:42, David Relson wrote:
> G'day,
>
> I'm porting the software for an embedded medical device from DOS to
> Linux and am wondering which file systems are appropriate and which are
> not.  The device's mass storage is a Disk-on-Module solid state flash
> drive.  Data is presently written at approx 100 bytes every 30 seconds
> but that might change to 100 bytes every second.  The device has a
> watchdog (recently activated) and during today's session it was
> triggered and wiped out my file system.
>
> Anybody have recommendations on which file system to use and the
> appropriate settings?
>
> Anybody have suggested readings so I can educate myself?
>

In addition to what everyone else has already noted you really need to 
state your tradeoff between lifetime of the flash cards and minimising 
the risk of loosing data.  Fundamentally you can buffer the data and 
write less frequently to the device (your device has a finite number of 
writes before it perishes), or you can write more frequently, hence less 
risk of dataloss

Any journal'ed fs is *supposed* to be reasonably robust against disk 
crashes during writes, but often there are layered levels of safety, eg 
ext3 has three write modes depending how you want to trade safety for 
speed (hint newest defaults are probably not optimal if your data is 
very precious)

I think one of the issues you will face is that whilst a journaling fs 
with appropriate options should be very safe against you flipping the 
power off whenever you feel like, long term your failure mode with flash 
appears to be random files disappearing and getting corrupted - the wear 
levelling appears to mean that unrelated files can get trashed as the 
flash shuffles data around to maximise wear levels until the card 
basically pretty much gives up?  I haven't experienced this yet...

I guess just plan ahead for this.

Good luck

Ed W