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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.


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