lkml.org 
[lkml]   [2007]   [Jun]   [27]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [RFC] fsblock
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...
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/

\
 
 \ /
  Last update: 2007-06-28 04:47    [W:0.130 / U:0.072 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site