Subject: Re: MTD devices in NetBSD
To: Garrett D'Amore <>
From: Terry Moore <>
List: tech-kern
Date: 03/23/2006 14:59:37
Hi Garret,

My point was that to have a common layer, you have to design for 
NAND, not for NOR, and that it's not something anyone can do casually.

Based on my experience, trying to couple this kind of detail into a 
file system is a really bad idea.  FTLs (the generic term in the 
industry for "Flash Transfer Layers", the transfer layers that hide 
the media from the file system) are much better.  The commercial 
market has come to this conclusion too; this is why CF cards don't 
expose the flash to the host machine.  In the early 90s, I helped 
write the PCMCIA spec that introduced standards for all this, and 
worked for a company that used both Microsoft's original FFS and then 
M-Systems TFFS (which is really an FTL).  Microsoft's original FFS 
never worked well. The problem was that the FFS systems that knew 
about the media details were much too complex, and were 
unmaintainable as the media evolved.  It is possible to get 
everything (including performance and reliability) via an FTL; then 
you can use existing file systems (FAT, UDF, etc).  There really 
aren't enough hours in the day to deal with all these details.  I've 
written file system and FTL code, and the thought of trying to test 
all the possible interactions between the file system, FTL and memory 
technology is daunting.  Since the performance of the file system is 
critical for system integrity and security, keeping things simple is 
a really good idea.  In addition to all their other properties, FTLs 
have a really clean unit-test interface.

Somewhat off topic, "MTD" is the term of art that was invented by 
PCMCIA to describe the low level drivers for memory access (SRAM 
cards, EPROM cards, Flash cards).  Five years from now something may 
well replace "flash", but wear levelling and data integrity 
considerations that are peculiar to non-rotating inexpensive media 
are likely still to be an issue.   I'd strongly suggest using "mtd" 
rather than "flash" as the name of the bus, for this reason.  If I 
understood the earlier comments, Linux got this one right.


At 11:45 AM 3/23/2006 -0800, Garrett D'Amore wrote:
>I *think* the existence of
>the MTD framework in Linux is evidence that the notion that the
>abstractions have *something* in common is not totally insane.