ext4 writepages is making tiny bios?

[Christoph Hellwig]

On Thu, Sep 03, 2009 at 03:52:01PM +1000, Dave Chinner wrote:
> > XFS did the mistake of trusting the VM, while everyone more or less
> > overrode it.  Removing all those checks and writing out much larger
> > data fixes it with a relatively small patch:
> > 
> > 	http://verein.lst.de/~hch/xfs/xfs-writeback-scaling
> 
> Careful:
> 
> -	tloff = min(tlast, startpage->index + 64);
> +	tloff = min(tlast, startpage->index + 8192);
> 
> That will cause 64k page machines to try to write back 512MB at a
> time. This will re-introduce similar to the behaviour in sles9 where
> writeback would only terminate at the end of an extent (because the
> mapping end wasn't capped like above).

Pretty good point, any applies to all the different things we discussed
recently.  Ted, should be maybe introduce a max_writeback_mb instead of
the max_writeback_pages in the VM, too?