linux-kernel - Re: Compact Flash Question

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Date:	Wed, 07 May 2008 14:31:43 +0200
From:	Helge Hafting <helge.hafting@...el.hist.no>
To:	Tomasz Chmielewski <mangoo@...g.org>
CC:	Bart Van Assche <bart.vanassche@...il.com>,
	LKML <linux-kernel@...r.kernel.org>, YSadgat1@...e.com,
	linux-os@...logic.com, Alan <alan@...rguk.ukuu.org.uk>,
	Linux IDE <linux-ide@...r.kernel.org>
Subject: Re: Compact Flash Question

Tomasz Chmielewski wrote:
>
> How does it work, then?
> How can it do wear levelling over the whole CF if some (or most) area 
> of CF is already used by our precious data/metadata?
> It would have to know the areas where no data is stored, but it 
> contradicts the CF <-> filesystem separation.
It don't necessarily need to know. It can swap two used blocks, one 
often-used and one
rarely-used.  That way the rarely-used block is rewriten over the 
previously busy
block, and the busy block is moved to the rarely used area that isn't worn.
This implies an extra write whenever a busy block is moved.  Don't know if
anybody do this, but the technique is simple enough.

CF-filesystem separation is necessary, for they can't know in advance what
filesystem or partitioning scheme will be used.  (I have ext3 on CF, for 
example...)

Helge Hafting


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