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SubjectRe: [prepatch] address_space-based writeback
On Apr 11, 2002  14:27 -0700, Andrew Morton wrote:
> One thing I'm not clear on with the private metadata address_space
> concept: how will it handle blocksize less than PAGE_CACHE_SIZE?
> The only means we have at present of representing sub-page
> segments is the buffer_head. Do we want to generalise the buffer
> layer so that it can be applied against private address_spaces?
> That wouldn't be a big leap.

I was going to send you an email on this previously, but I (think I)
didn't in the end...

At one time Linus proposed having an array of dirty bits for a page,
which would allow us to mark only parts of a page dirty (say down to
the sector level). I believe this was in the discussion about moving
the block devices to the page cache around 2.4.10.

While that isn't a huge win in the most cases (it costs the same to
write 512 bytes as 4096 bytes to disk because of disk latencies) it may
be more important if/when we ever have larger pages. This also becomes
more important if you are working with a network filesystem where you
have to send all of the dirty data over a much smaller pipe, so sending
512 bytes takes 1/8 as long as sending 4096 bytes.

Cheers, Andreas
--
Andreas Dilger
http://www-mddsp.enel.ucalgary.ca/People/adilger/
http://sourceforge.net/projects/ext2resize/

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