Re: [00/17] Large Blocksize Support V3

[Alan Cox <alan@lxorguk.ukuu.org.uk>]

> > 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.

Alan
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