Re: [RFC] basic delayed allocation in VFS

[David Chinner <dgc@sgi.com>]

On Sun, Jul 29, 2007 at 04:09:20PM +0400, Alex Tomas wrote:
> David Chinner wrote:
> >On Fri, Jul 27, 2007 at 11:51:56AM +0400, Alex Tomas wrote:
> >But this is really irrelevant - the issue at hand is what we want
> >for VFS level delalloc support. IMO, that mechanism needs to support
> >both XFS and ext4, and I'd prefer if it doesn't perpetuate the
> >bufferhead abuses of the past (i.e. define an iomap structure
> >instead of overloading bufferheads yet again).
> 
> I'm not sure I understand very well.

->get_blocks abuses bufferheads to provide an offset/length/state
mapping. That's all it needs. That what the iomap structure is used
for. It's smaller than a bufferhead, it's descriptive of it's use
and you don't get it confused with the other 10 ways bufferheads
are used and abused.

> where would you track uptodate, dirty and other states then?
> do you propose to separate block states from block mapping?

No. They still get tracked in the bufferheads attached to the page.
That's what bufferheads were originally intended for(*).

Cheers,

Dave.

(*) I recently proposed a separate block map tree for this rather
than using buffer heads for this because of the memory footprint of
N bufferheads per page on contiguous mappings. That's future work,
not something we really need to consider here. Chris Mason's extent
map tree patches are a start on this concept.
-- 
Dave Chinner
Principal Engineer
SGI Australian Software Group