buffered writeback torture program

[Chris Mason]

Excerpts from Christoph Hellwig's message of 2011-04-21 14:08:05 -0400:
> On Thu, Apr 21, 2011 at 02:02:43PM -0400, Chris Mason wrote:
> > For this program, they are almost all dirty pages.
> > 
> > I tried patching it to give up if we seek but it is still pretty slow.
> > There's something else going on in addition to the xfs clustering being
> > too aggressive.
> 
> I'm not sure where you this beeing to agressive from - it's doing
> exactly the same amount of I/O as a filesystem writing out a single
> page from ->writepage or using write_cache_pages (either directly
> or as a copy) as ->writepages.  The only thing special compared to
> the no ->writepages case is that it's submitting a large I/O
> from the first ->writepage call.
> 

Ok, I see what you mean.  The clustering code stops once it hits
nr_to_write, I missed that.  So we shouldn't be doing more than a single
writepages call.

-chris