Flash File System

[Robin Kirkham]

> I need a Flash File System for a project i am building with the RTEMS. I
> will attempt to create this so that it can be incorporated into the project
> as a whole.
> 
> 1. Is there anything i should be aware of before i do this. Especially with
> regard to submitting the code for inclusion in future versions of RTEMS.
> ...

I don't have anything that will help Steven directly (although I once fiddled
with some code that built an MS-DOS filesystem in a battery-backed up RAM
unit--this was on vxWorks which already has all the filesystem code).

It does raise an interesting point, however, that of filesystems and RTEMS:
it has been talked about before on this list.

Usually, small kernels like RTEMS and vxWorks have a device table of some
kind, with name entries which link to driver code. This is fine for a small
number of entries--such as /dev/console, /dev/tty0, and so on. When a "file"
is open()ed, the name is matched against each entry in turn. The / character
has no particular significance, except (at least in vxWorks) if it is missing
from the file name to be opened, another string, the "current directory" is
prepended before the search.

The problem comes when real file-systems come along. vxWorks has a number of
these--RT11, MS-DOS, NFS and a few others by now I expect. These are integrated
into the driver-name scheme described above. It works well enough, until
someone expects UNIX-filesystem semantics to apply throughout, and tries to
open "/fred/../dev/tty0". Or expects an ls-like program to work in / or /dev.

I think RTEMS can do a little better. What is needed is a virtual,
tree-structured memory filesystem, with all the proper semantics (i.e.,
/, ., and .. work, etc). I imagine directory entries would reference vnodes
of some kind, which would reference driver major/minor numbers, or inodes
of a real filesystem.

In other words, I think there should be a consistent structure onto which
drivers for real file-systems, such as flash cards, NFS (given we have ka9q,
it's not a big step to port RPC and NFS) and, lord help us, real disk drives,
can be mounted. The structure really should be in place *before* these other
filesystem come along.

Just my $0.02 cents worth (actually, $0.017 given the current state of AUD).

Cheers,

Robin Kirkham			CSIRO Manufacturing Science and Technology
Project Engineer		Locked Bag 9, Preston 3072, Australia
robin.kirkham at mlb.dmt.csiro.au	Phone: +61 3 9662-7756  Fax: +61 3 9662-7851