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    SubjectRe: Scatter-gather list constraints
    On Sun, 22 Jun 2008, Andi Kleen wrote:

    > >> - Is it performance critical?
    > >
    > > For people using wireless USB drives, yes.
    > But only if there is a lot of 512 byte block IO? The only case I can think
    > of right now would be XFS log IO and perhaps some O_DIRECT/raw device
    > accesses.

    Sorry, I misunderstood your question. There probably will not be a lot
    of 512-byte block I/O -- not in the workloads I'm acquainted with. But
    there will be some.

    It isn't performance-critical, in the sense that slowing down the odd
    512-byte block transfers won't hurt performance much. But it is
    critical in the sense that the transfers must work properly when they
    do occur.

    > If it's only an relative oddball just copying is fine imho.

    You mean, have the USB stack allocate bounce buffers and copy the data
    between the S-G buffers (which may be in high memory) and the bounce
    buffers? We're talking about a potentially fairly large amount of
    data, say up to 100 KB. Is that really easier than splitting up an I/O

    Alan Stern

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