Re: [RFC] fsblock

[David Chinner <dgc@sgi.com>]

On Wed, Jun 27, 2007 at 07:50:56AM -0400, Chris Mason wrote:
> On Wed, Jun 27, 2007 at 07:32:45AM +0200, Nick Piggin wrote:
> > On Tue, Jun 26, 2007 at 08:34:49AM -0400, Chris Mason wrote:
> > > On Tue, Jun 26, 2007 at 07:23:09PM +1000, David Chinner wrote:
> > > > On Tue, Jun 26, 2007 at 01:55:11PM +1000, Nick Piggin wrote:
> > > 
> > > [ ... fsblocks vs extent range mapping ]
> > > 
> > > > iomaps can double as range locks simply because iomaps are
> > > > expressions of ranges within the file.  Seeing as you can only
> > > > access a given range exclusively to modify it, inserting an empty
> > > > mapping into the tree as a range lock gives an effective method of
> > > > allowing safe parallel reads, writes and allocation into the file.
> > > > 
> > > > The fsblocks and the vm page cache interface cannot be used to
> > > > facilitate this because a radix tree is the wrong type of tree to
> > > > store this information in. A sparse, range based tree (e.g. btree)
> > > > is the right way to do this and it matches very well with
> > > > a range based API.
> > > 
> > > I'm really not against the extent based page cache idea, but I kind of
> > > assumed it would be too big a change for this kind of generic setup.  At
> > > any rate, if we'd like to do it, it may be best to ditch the idea of
> > > "attach mapping information to a page", and switch to "lookup mapping
> > > information and range locking for a page".
> > 
> > Well the get_block equivalent API is extent based one now, and I'll
> > look at what is required in making map_fsblock a more generic call
> > that could be used for an extent-based scheme.
> > 
> > An extent based thing IMO really isn't appropriate as the main generic
> > layer here though. If it is really useful and popular, then it could
> > be turned into generic code and sit along side fsblock or underneath
> > fsblock...
> 
> 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.

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......

> I do think fsblocks is a nice cleanup on its own, but Dave has a good
> point that it makes sense to look for ways generalize things even more.

*nod*

Cheers,

Dave.
-- 
Dave Chinner
Principal Engineer
SGI Australian Software Group