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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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