Subject: Re: MTD devices in NetBSD
To: Charles M. Hannum <mycroft@MIT.EDU>
From: David Brownlee <>
List: tech-kern
Date: 03/23/2006 18:06:20
On Thu, 23 Mar 2006, Charles M. Hannum wrote:

> On Wed, Mar 22, 2006 at 06:45:08PM +0000, David Brownlee wrote:
>> 	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).
> Unfortunately, current file systems like FFS can't do this, because they
> zero all the unused data in a block, and rewrite it every time.  This is
> a good example of why a flash-aware file system is important -- you can
> save a lot of wear, and gain a lot performance here.

 	Presumably that could be handled by having the layer
 	underneath the filesystem test if a write only sets bits
 	in an existing block, and if so just go ahead and 'overwrite'
 	that block. It still doesn't make existing filesystems a good
 	fit to flash devices...

 		David/absolute       -- No hype required --