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    Subject[PATCH 003 of 6] md: Define raid5_mergeable_bvec

    This will encourage read request to be on only one device,
    so we will often be able to bypass the cache for read
    requests.

    cc: "Raz Ben-Jehuda(caro)" <raziebe@gmail.com>
    Signed-off-by: Neil Brown <neilb@suse.de>

    ### Diffstat output
    ./drivers/md/raid5.c | 24 ++++++++++++++++++++++++
    1 file changed, 24 insertions(+)

    diff .prev/drivers/md/raid5.c ./drivers/md/raid5.c
    --- .prev/drivers/md/raid5.c 2006-10-31 16:40:57.000000000 +1100
    +++ ./drivers/md/raid5.c 2006-10-31 16:41:26.000000000 +1100
    @@ -2611,6 +2611,28 @@ static int raid5_congested(void *data, i
    return 0;
    }

    +/* We want read requests to align with chunks where possible,
    + * but write requests don't need to.
    + */
    +static int raid5_mergeable_bvec(request_queue_t *q, struct bio *bio, struct bio_vec *biovec)
    +{
    + mddev_t *mddev = q->queuedata;
    + sector_t sector = bio->bi_sector + get_start_sect(bio->bi_bdev);
    + int max;
    + unsigned int chunk_sectors = mddev->chunk_size >> 9;
    + unsigned int bio_sectors = bio->bi_size >> 9;
    +
    + if (bio_data_dir(bio))
    + return biovec->bv_len; /* always allow writes to be mergeable */
    +
    + max = (chunk_sectors - ((sector & (chunk_sectors - 1)) + bio_sectors)) << 9;
    + if (max < 0) max = 0;
    + if (max <= biovec->bv_len && bio_sectors == 0)
    + return biovec->bv_len;
    + else
    + return max;
    +}
    +
    static int make_request(request_queue_t *q, struct bio * bi)
    {
    mddev_t *mddev = q->queuedata;
    @@ -3320,6 +3342,8 @@ static int run(mddev_t *mddev)
    mddev->array_size = mddev->size * (conf->previous_raid_disks -
    conf->max_degraded);

    + blk_queue_merge_bvec(mddev->queue, raid5_mergeable_bvec);
    +
    return 0;
    abort:
    if (conf) {
    -
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