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Date:      Thu, 25 May 2006 14:37:27 -0400
From:      "Andrew Atrens" <atrens@nortel.com>
To:        Olivier Gautherot <olivier@gautherot.net>
Cc:        Alexander Leidinger <Alexander@Leidinger.net>, Poul-Henning Kamp <phk@phk.freebsd.dk>, small@freebsd.org, current@freebsd.org
Subject:   Re: FreeBSD's embedded agenda
Message-ID:  <4475F967.5040806@nortel.com>
In-Reply-To: <1148580598.4475f2f677197@imp2-g19.free.fr>
References:  <3981.1148578569@critter.freebsd.dk> <4475EFC1.1020504@nortel.com> <1148580598.4475f2f677197@imp2-g19.free.fr>

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Hi Olivier,

Olivier Gautherot wrote:
> Hi Andrew!
> 
> 
>>[...]
>>
>>>The reason Flash Adaptation Layers came about in the first place
>>>is that W95 didn't support anything but FAT.
>>
>>
>>Hmm. I was thinking about partitioning the problem actually. Make flash
>>look like a disk and then you can put any filesystem on it that you
>>want. Seems a heck of a lot simpler .. and I'm not sure if I see any
>>drawbacks to doing it that way ...
> 
> 
> The drawback is the following: what would happen if you had an application
> opening-writing-closing a file in /var/log on a regular basis? The block
> would decay with time, with chances that your log even gets corrupted.
> That's why Flash drivers have to spread write accesses across the device
> (what FFS doesn't naturally do). Also, there is a constraint regarding
> the changes allowed: on NAND flash, you can write a 0 on a bit but have
> to erase the full block to write a 1 back.
> 
> Don't forget that Flash doesn't suffer from mechanical delays so there
> is no harm in fragmenting the filesystem: this would be another feature.
> 
> My cent worth ;-)

Yes, exactly... that's precisely what 'wear-leveling' is meant to do ..

I think I mentioned wear-leveling further back in the email chain ..

Yes, you definitely want wear-leveling. The debate is whether the
filesystem knows about it, versus it being managed by a lower level
'driver'.

Andrew.
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