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    SubjectRe: [PATCH] 2.4.x write barriers (updated for ext3)
    On Mon, Mar 04 2002, Daniel Phillips wrote:
    > But the bio layer can manage it, by sending a write barrier down all relevant
    > queues. We can send a zero length write barrier command, yes?

    Actually, yes that was indeed one of the things I wanted to achieve with
    the block layer rewrite -- the ability to send down other commands than
    read/write down the queue. So not exactly bio, but more of a new block

    See, now fs requests have REQ_CMD set in the request flag bits. This
    means that it's a "regular" request, which has a string of bios attached
    to it. Doing something ala

    struct request *rq = get_request();

    rq->rq_dev = target_dev;
    rq->cmd[0] = GPCMD_FLUSH_CACHE;
    rq->flags = REQ_PC;
    /* additional info... */

    would indeed be possible. The attentive reader will now know where
    ide-scsi is headed and why :-)

    This would work for any SCSI and psueo-SCSI device, basically all the
    stuff out there. For IDE, the request pre-handler would transform this
    into an IDE command (or taskfile).

    Jens Axboe

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