[H-GEN] Flash drives and wear levelling: user experiences?

[Clinton Roy]

Hi,

> My only worry at this stage, is how to go about installing the OS on the compact flash card, and configuring everything in such a way, that I don't burn holes in the CF card.  Some CF cards allegedly have hardware wear-levelling, which is good news (but slooow), but it seems like a good idea to try to set everything up so that the amount of writing to the CF card is minimised.

I hear that too, but i'm yet to actually see any card that has hit its
write limit. I tend to fry them, melt them, snap them, smash them or
just simply lose them quicker than 100k writes.

> My question to the HUMBUG collective is; has anybody else gotten any significant hands-on experience with working with cut-down machines running off flash drives, and want to tell us their experiences, and what they think is the best way to arrange the filesystem, etc to work well with flash media?

The simplest way is to mount root read only and use some tmpfs mounts
for things that need writing to, like /tmp, /var and /etc

Some rc scripts already have support for this, debian unstable rc
listens to directives like TMPVARLOCK and TMPVARLOG from
/etc/default/rc

Some mount commands seem to be able to share the namespace, under
linux it's simple enough to use unionfs:

tmpfs /temporary/var
union /var directories=/temporary/var:/var

will make it such that reads drop through to /var on the compact
flash, but writes only happen on the tmpfs filesystem. It's possible
to set  all this up just from fstab without having any special scripts
anywhere.

later,
-- 
Clinton Roy
CSIRO - Robotics Platform Engineer
Autonomous Systems Lab

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