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    SubjectRe: Bio pool & scsi scatter gather pool usage
    Andrew Morton wrote:

    >Alan Cox wrote:
    >
    >>>Perhaps, but calls are expensive. Repeated calls down stacked block
    >>>devices will add up. In only the most unusually cases will there
    >>>
    >>You don't need to repeatedly query. At bind time you can compute the
    >>limit for any device heirarchy and be done with it.
    >>
    >
    >S'pose so. The ideal request size is variable, based
    >on the alignment. So for exampe if the start block is
    >halfway into a stripe, the ideal BIO size is half a stripe.
    >
    >But that's a pretty simple table to generate once-off,
    >as you say.
    >

    But this gets you lowest common denominator sizes for the whole
    volume, which is basically the buffer head approach, chop all I/O up
    into a chunk size we know will always work. Any sort of nasty boundary
    condition at one spot in a volume means the whole thing is crippled
    down to that level. It then becomes a black magic art to configure a
    volume which is not restricted to a small request size.

    Steve


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