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    SubjectRe: [00/17] Large Blocksize Support V3
    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.


    Dave Chinner
    Principal Engineer
    SGI Australian Software Group
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