Re: [RFC] fsblock

[Nick Piggin <npiggin@suse.de>]

On Thu, Jun 28, 2007 at 08:35:48AM +1000, David Chinner wrote:
> On Wed, Jun 27, 2007 at 07:50:56AM -0400, Chris Mason wrote:
> > Lets look at a typical example of how IO actually gets done today,
> > starting with sys_write():
> > 
> > sys_write(file, buffer, 1MB)
> > for each page:
> >     prepare_write()
> > 	allocate contiguous chunks of disk
> >         attach buffers
> >     copy_from_user()
> >     commit_write()
> >         dirty buffers
> > 
> > pdflush:
> >     writepages()
> >         find pages with contiguous chunks of disk
> > 	build and submit large bios
> > 
> > So, we replace prepare_write and commit_write with an extent based api,
> > but we keep the dirty each buffer part.  writepages has to turn that
> > back into extents (bio sized), and the result is completely full of dark
> > dark corner cases.

That's true but I don't think an extent data structure means we can
become too far divorced from the pagecache or the native block size
-- what will end up happening is that often we'll need "stuff" to map
between all those as well, even if it is only at IO-time.

But the point is taken, and I do believe that at least for APIs, extent
based seems like the best way to go. And that should allow fsblock to
be replaced or augmented in future without _too_ much pain.

 
> Yup - I've been on the painful end of those dark corner cases several
> times in the last few months.
> 
> It's also worth pointing out that mpage_readpages() already works on
> an extent basis - it overloads bufferheads to provide a "map_bh" that
> can point to a range of blocks in the same state. The code then iterates
> the map_bh range a page at a time building bios (i.e. not even using
> buffer heads) from that map......

One issue I have with the current nobh and mpage stuff is that it
requires multiple calls into get_block (first to prepare write, then
to writepage), it doesn't allow filesystems to attach resources
required for writeout at prepare_write time, and it doesn't play nicely
with buffers in general. (not to mention that nobh error handling is
buggy).

I haven't done any mpage-like code for fsblocks yet, but I think they
wouldn't be too much trouble, and wouldn't have any of the above
problems...