Re: [RFC] MTD driver for MMC cards

David Woodhouse
2007-01-02T00:08:22Z

On Mon, 2007-01-01 at 23:22 +0100, Arnd Bergmann wrote:
> There are multiple efforts in progress to get a jffs2 replacement. NAND
> flash in embedded devices has the same size as it has on MMC card
> potentially, so we will need one soon. David Woodhouse has pushed the
> limit that jffs2 can reasonably used to 512MB, which is the size used
> in the OLPC XO laptop. If there are ways to get beyond that (which I
> find unlikely), there will be a hard limit 2GB or 4GB because of
> limitations in the fs layout.

The main weakness of JFFS2 (at this kind of size) is that there _is_ no
fs layout -- so there isn't a hard 2GiB or 4GiB limit in the format,
because we never encode offsets anywhere but in memory.

We'll push JFFS2 further than the current 512MiB by enlarging the data
nodes -- so each node covers something like 16KiB of data instead of
only 4KiB, and then there'll be about 1/3 as many of them, which will
cut the memory usage and reduce the amount we need to read in the
"summary" blocks. But logfs is the way forward, I agree.

--
dwmw2