Re: [ECOS] Simple flash filesystem?

["Lewin A.R.W. Edwards" <larwe at larwe dot com>]

Hi Grant,

>I've been looking for info on flash filesystems, and have found pretty
>much nothing.

Random notes:

I suspect that the reason you haven't found many references is that these 
sorts of applications (in my experience so far) fall into these categories:

1. Cases where you _want_ to emulate a DOS-type filesystem, especially 
cases where you have an underlying flash controller that does the 
level-wearing and error correction for you, and

2. Cases where you're only going to write data once in a blue moon, and 
it's most efficient to solve the problem on an ad hoc basis.

It really doesn't matter what logical sector size you use as long as you 
have a reasonably efficient scatter/gather system. I have a very very 
similar problem working on SmartMedia - the filesystem is organized into 
512-byte sectors but the card is only erasable in blocks (which by default 
are cluster-sized). In order to keep the upper layers of the filesystem as 
generic as possible, I let my DOS filesystem work with the 512-byte sectors 
it knows and loves.

In older versions of my SSFDC code, the DOS layer decomposed each R/W op 
into a series of single sector R/W ops. In the case of a random sector W 
op, the flash interface layer would then read in a block, update, erase and 
verify. This was obviously very inefficient (though it works fine on 
CompactFlash, which has an intelligent R/W controller), so I later changed 
the breakdown. In my current code, the DOS filesystem layer decomposes R/W 
ops into a series of variable-length ops, each of which is no more than one 
cluster long. This implicitly allows the underlying flash driver to make 
the R/W/erase loop much more efficient.

By judiciously adding a few extra K of RAM, it is also possible to increase 
the write caching capability to cope with any erase block size.

The main performance hit I encountered in emulating a DOS filesystem over 
dumb flash is updating the FAT. If you can keep the whole FAT, or at least 
all the sectors for open-for-writing chains, in RAM (and only update 
sections when open-for-write files are closed), you'll have a huge 
performance increase.

In the eval board for the processor in our old products, the vendor uses a 
very simple, dumb, intended-for-read-only filesystem. It's easiest to 
illustrate by example: If your flash free space starts at 0x20000:

0x20000 00 12 00 00  = length of file (x00001200), 0x00000000 for deleted 
file, or 0xFFFFFFFF if no more files
0x20004 FREDXXXX.XXX\0 = filename (ASCIIZ)
0x2000x file data
0x2120x next file header

To find a particular file you start at the beginning of filespace and get 
the first word (pointer to next file). You then check the filename 
immediately after the word. If it's the file you want, then read it out. If 
it's not the desired file, then use the pointer to skip to the next file 
until you find the one you want or reach the end of the chain. Each file is 
guaranteed (by manipulating the length field) to start on a write-page 
boundary (not an erase-block boundary though).

If you do something like this, you will eventually get so fragmented that 
you'll need to gather up all the files again (which you could do with an 
"optimize" option in your UI if you wanted to). But as a quick and dirty 
solution, it has some merit.

Maybe you might want to consider NAND flash (SmartMedia in a chip package, 
essentially) for your file storage. It does have the benefit of having a 
block size that is exactly the same as the best DOS cluster size for the 
media capacity. And it's fairly easy to work with.

=== Lewin A.R.W. Edwards (Embedded Engineer)
Work: http://www.digi-frame.com/
Personal: http://www.zws.com/ and http://www.larwe.com/

"Und setzet ihr nicht das Leben ein,
Nie wird euch das Leben gewonnen sein."