FFS2 and MTDs (flash)

David Woodhouse David.Woodhouse at mvhi.com
Mon Jul 5 15:06:38 EDT 1999

jgg at ualberta.ca said:
>  Yes, there is lots of benifit to being able to mount a FFS filesystem
> over a loop device or on a RAM disk or something else. There is also
> lots of benifit to being able to create the FFS off the flash and then
> write the whole thing in one go. For my purposes that is a crucial
> feature. Also since the MTD's are presented as block devices you can
> mount other FS's like romfs in read-only mode.  

OK - that sounds reasonable.

jgg at ualberta.ca said:
>  Things will also run a bit faster if the buffer cache is used to hang
> onto some often used filesystem meta data in main memory.

Think of CFI-compliant flash mapped into the host's address space. Why cache 
it in RAM when you can just point a page table at the original?

> XIP is not directly possible with FFS2 as it places no constraints on
> alignment (like romfs).

So what they say on their web site about XIP is all crap? Of course, I suppose 
that shouldn't surprise me :)

Is it possible to run Linux on FFS2 without some kind on UMSDOS-like 
translation layer? Are we going to want to write our own version which is 

>  You're welcome to my code so far if you are interested, I have
> support for an Octagon 5066, a V-Max 301 and untested support for
> something called a MixCom card. 

Yes please - that'd be useful.

> BTW, I thought Disk on a chip used an ATA interface, and wasn't really
> a MTD? 

No, it maps into the host's memory map between 0xD0000 and 0xE0000 and has a 
custom ASIC to interface to the flash. The old versions behaved very similarly 
to EMS, paging in different blocks of flash - but the new version is very 
different, and very strange. 

It's CompactFlash that uses an ATA interface, either PCMCIA-ATA or raw IDE. So
you don't even need PCMCIA support to use it, I believe.

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