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