Re: [patch 10/21] buffer heads: Support slab defrag

[Christoph Lameter <clameter@sgi.com>]

On Fri, 16 May 2008, David Chinner wrote:

> On Thu, May 15, 2008 at 10:42:15AM -0700, Christoph Lameter wrote:
> > On Mon, 12 May 2008, David Chinner wrote:
> > 
> > > If you are going to clean bufferheads (or pages), please clean entire
> > > mappings via ->writepages as it leads to far superior I/O patterns
> > > and a far higher aggregate rate of page cleaning.....
> > 
> > That brings up another issue: Lets say I use writepages on a large file 
> > (couple of gig). How much do you want to write back?
> 
> We're out of memory. I'd suggest write backing as much as you can
> without blocking.  e.g. treat it like pdflush and say 1024 pages, or
> like balance_dirty_pages() and write a 'write_chunk' back from the
> mapping (i.e.  sync_writeback_pages()).

Why are we out of memory? How do you trigger such a special writeout?
 
> Any of these are better from an I/O perspective than single page
> writeback....

But then filesystem can do tricks like writing out the surrounding areas 
as needed. The filesystem likely can estimate better how much writeout 
makes sense.