Re: [00/17] Large Blocksize Support V3

[Andrew Morton <akpm@linux-foundation.org>]

On Sat, 28 Apr 2007 11:21:17 +0100 Alan Cox <alan@lxorguk.ukuu.org.uk> wrote:

> > > Also remember that even if you do larger pages by using virtual pairs or
> > > quads of real pages because it helps on some systems you end up needing
> > > the same sized sglist as before so you don't make anything worse for
> > > half-assed controllers as you get the same I/O size providing they have
> > > the minimal 2 or 4 sg list entries (and those that don't are genuinely
> > > beyond saving and nowdays very rare)
> > > 
> > 
> > Could you expand on that a bit please?  I don't get it.
> 
> Put a 16K "page" into the page cache physically and you need to allocate
> 1 sg entry and you get a clear benefit, IFF you can allocate the pages. 
> 
> Put a 16K "page" into the page cache made up of 4 x real 4K pages which
> are not physically contiguous and you need 4 sg list entries - which is
> no worse than if you were using 4K pages
> 
> 	4 per 16K page cache "logcial page"		-> 4 per 16K
> 
> 	1 per 4K physical page for 4K page cache	-> 4 per 16K
> 
> The only ugly case for the latter is if you are reading something like a
> 16K page ext3fs from an old IA64 box onto a real computer and you have a
> controller with insufficient sg list entries to read a 16K logical page.
> At that point the block layer is going to have kittens.
> 

OK.

But all (both) the proposals we're (ahem) discussing do involve 4x
physically contiguous pages going into those four contiguous pagecache
slots.

So we're improving things for the half-assed controllers, aren't we?