buffered writeback torture program

[Christoph Hellwig]

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.