First cut at MTD/JFFS HOWTO

David Woodhouse dwmw2 at
Tue Feb 13 04:15:47 EST 2001

vmalik at said:
> Q. Why another file system. What was wrong with ext2? 
> A. 

Standard response - need journalling pseudo-filesystem to emulate a block 
device and to wear levelling. then need ext3 (note 3) on that. journalling 
fs on top of journalling fs - not efficient. Also, no way for ext[23] to 
mark blocks as _deleted_ and no longer cared about. Fill ext2 partition on 
NFTL, empty it again, and the NFTL will still carefully copy around the 
blocks containing old deleted data.

> Q. Do I have to have JFFS on MTD? 
> A. 

ATM yes. Once you could do it on a block device. People are talking about 
making me make it work on IDE devices (CF). But I don't want to :)

> Q. What is DOC (disk on chip)? A. 

Bunch of NAND flash chips connected together with a clever ASIC which does 
hardware ECC.

> Q. What File systems can I have on DOC? A. 

If you put NFTL on it to emulate a block device (the status quo) then any 
normal filesystem. JFFS ought to work too.

> Q. What is Flash memory? A. 
> Q. What is CFI Flash memory? A. 
> Q. What is JDEC Flash memory? A.
> Q. What is this "interleave" stuff? A.

If you have 16-bit chips, but a 32-bit processor, it makes sense to arrange 
them side-by-side to fill the CPU's bus. You drive them both 
simultaneously. That's the arrangement we refer to as 'interleave'. 

Other possibilities are... 4x 8-bit chips on 32-bit bus, 2x8-bit 
chips on 16-bit bus, ...

#include <compact_flash_is_not_flash.h>


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