Re: [00/17] Large Blocksize Support V3

David Chinner
2007-04-26T14:18:52Z

On Thu, Apr 26, 2007 at 01:37:42PM +0100, Andy Whitcroft wrote:
> Nick Piggin wrote:
> > Christoph Lameter wrote:
> >> On Thu, 26 Apr 2007, Nick Piggin wrote:
> >>>> What are the exact requirement you are trying to address?
> >>>
> >>> Block size > page cache size.
> >>
> >> But what do you mean with it? A block is no longer a contiguous
> >> section of memory. So you have redefined the term.
> >
> > I don't understand what you mean at all. A block has always been a
> > contiguous area of disk.
>
> Lets take Nick's definition of block being a disk based unit for the
> moment. That does not change the key contention here, that even with
> hardware specifically designed to handle 4k pages that hardware handles
> larger contigious areas more efficiently. David Chinner gives us
> figures showing major overall throughput improvements from (I assume)
> shorter scatter gather lists and better tag utilisation.

I haven't actually provided any figures - it's knowledge passed down
from those that know more about it that I do.

If you want figures about the impact of large I/Os, then we should
not be looking at the HBAs but at the impact on RAID controller
throughput (this I do have numbers on ;). It is not uncommon to
see 2MB I/Os give twice the throughput of 512K I/Os to a single
RAID controller - larger than 512k can only be acheived on systems
with a page size larger than 4k.....

> I am loath to
> say we can just blame the hardware vendors for poor design.

Never did - I'm pointing out that linux can't use all the
capabilities they have.

Cheers,

Dave.
--
Dave Chinner
Principal Engineer
SGI Australian Software Group