Subject: Re: [RFC] MTD driver for MMC cards
From: David Woodhouse <dwmw2@infradead.org>
To: Arnd Bergmann <arnd@arndb.de>
Cc: Pierre Ossman <drzeus-mmc@drzeus.cx>, linux-mtd@lists.infradead.org,
       linux-kernel@vger.kernel.org,
       =?ISO-8859-1?Q?J=F6rn?= Engel <joern@wohnheim.fh-wedel.de>
In-Reply-To: <200701012322.14735.arnd@arndb.de>
References: <200612281418.20643.arnd@arndb.de> <4597ADD2.90700@drzeus.cx>
	 <200701012322.14735.arnd@arndb.de>
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Date: Tue, 02 Jan 2007 00:08:40 +0000
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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

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