Re: scsi command queuing

From: Oliver Rauch (oliver.rauch@Wolfsburg.DE)
Date: Sat Jul 01 2000 - 13:43:21 PDT

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    Douglas Gilbert wrote:

    >
    > Some info on Linux sg buffer sizes. DMA (used by most but not the very low
    > end SCSI adapters) requires continuous buffers. ["continuous" seen from the
    > PCI/ISA bus.] Linux actively discourages drivers taking big continuous
    > buffers. So sg takes 32 KB chunks (controlled by the SG_SCATTER_SZ define
    > in sg.h) and builds a scatter gather list. The adapter dictates the maximum
    > number of the scatter gather list elements which is further limited to
    > 255 elements. So that makes the maximum size of sg's reserve buffer
    > (or any single command) just under 8 Megabytes. You can double
    > SG_SCATTER_SZ, rebuild sg and perhaps get that number up to just under
    > 16 Megabytes.
    >

    Hi Douglas,

    how large may SG_SCATTER_SZ be made? Are there any limits?
    What side effects do happen if I increase it to e.g. 128KB ?

    Bye
    Oliver 

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