Subject: Re: MTD devices in NetBSD
To: Eduardo Horvath <firstname.lastname@example.org>
From: David Brownlee <abs@NetBSD.org>
Date: 03/22/2006 18:45:08
On Wed, 22 Mar 2006, Eduardo Horvath wrote:
> Time to ask annoying questions.
> On Tue, 21 Mar 2006, Garrett D'Amore wrote:
>> Okay, I'm ready to start getting *serious* about supporting MTD (flash!)
>> devices in NetBSD.
>> I've been doing a lot of research. I want to sum it up here, along
>> with my thoughts. I'm *particularly* interested in what core@ has to
>> say, so that perhaps we can move to actually implementing something
>> "real". I am aware that others are starting to work on this problem,
>> but I think there is a lack of hard technical direction, and I think it
>> is important to get the "framework" right:
>> MTD devices differ from regular "block" devices in some important aspects:
>> 1. you have to erase a block before you write it (read-modify-write
>> 2. They need wear-leveling (writes "wear" the bits out) to prolong
>> device life
>> 3. Many NOR devices can map directly into system memory while in read
>> 4. Generally NAND devices cannot do this, and usually need special
>> 5. You may have to to do bad block management (some block devices
>> have to do this)
>> 6. Generally, most filesystems designed for use with block devices
>> won't work so well
>> 7. Some devices support "execute in place", while some do not
>> 8. CompactFlash doesn't count, because it looks like an IDE disk. :-)
> How does the above differ from `regular "block" devices'?
> The erasure issue is only visible if you allow sub-block writes. Is that
> something you're relly thinking about doing? Is there any reason why you
> would want to leave a block erased without immediately writing useful data
> to it?
From a hardware perspective you should be able to perform
sub-block writes down to the word level. A perfect example
of that usage would be a an app performing synchronous
appending writes to a logfile. Providing you are not
_clearing_ any bit in the current block you can just 'write
over' the current values (assuming standard flash).
Its only during erase that the whole block becomes an issue.
David/absolute -- www.NetBSD.org: No hype required --