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    SubjectRe: direct (unbufferd) disk access
    On Thu, Jun 24, 1999 at 11:35:56AM +0200, Christian Hammers wrote:
    > Hello List
    > How can I get the kernel to read/write some data immediately to disk without
    > caching it in the kernel memory.
    > I like to access one harddrive with two motherboards and like to have a shared
    > partition that will be writen to by one motherboard and read by an other
    > motherboard - but Linux seems to prevent that.
    > I don't like to mount any partition (because of the filesystem check warnings)
    > and will instead write raw data with "dd".
    > Oh, and I don't like to flush ALL buffers, because this computer will be a
    > redunant web-server that strongly needs buffers (for this reasons AFAIK I
    > can't use flushb and sync).

    You may like to look at: [and follow the "timings" link]

    to get a feeling for what direct versus indirect IO is
    costing you. Reasonably standard i386 hardware can get
    you 20 to 30 MB/sec throughput using "indirect IO".
    For substantially faster throughput you need direct
    io. [These number are derived from the SCSI READ BUFFER
    command which bypasses physical IO on the disk.]

    You may also be interested in the utility "sg_dd512"
    which implements "dd" functionality using the sg
    device thus bypassing the block sub-system and its

    Doug Gilbert

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