path: root/drivers/md/dm-snap.c
diff options
authorMikulas Patocka <mpatocka@redhat.com>2010-08-12 04:14:02 +0100
committerAlasdair G Kergon <agk@redhat.com>2010-08-12 04:14:02 +0100
commitb1d5552838334c600b068c9c8cc18638e5a8cb47 (patch)
tree789e435ab60e6956abffc15b4da85fe1e5b1714c /drivers/md/dm-snap.c
parent4a0b4ddf261fc89c050fe0a10ec57a61251d7ac0 (diff)
dm snapshot: implement merge
Implement merge method for the snapshot origin to improve read performance. Without merge method, dm asks the upper layers to submit smallest possible bios --- one page. Submitting such small bios impacts performance negatively when reading or writing the origin device. Without this patch, CPU consumption when reading the origin on lvm on md-raid0 was 6 to 12%, with this patch, it drops to 1 to 4%. Note: in my testing, it actually degraded performance in some settings, I traced it to Maxtor disks having problems with > 512-sector requests. Reducing the number of sectors to /sys/block/sd*/queue/max_sectors_kb to 256 fixed the read performance. I think we don't have to care about weird disks that actually degrade performance because of large requests being sent to them. Signed-off-by: Mikulas Patocka <mpatocka@redhat.com> Signed-off-by: Alasdair G Kergon <agk@redhat.com>
Diffstat (limited to 'drivers/md/dm-snap.c')
1 files changed, 16 insertions, 0 deletions
diff --git a/drivers/md/dm-snap.c b/drivers/md/dm-snap.c
index a1f2ab553b9..96feada5e76 100644
--- a/drivers/md/dm-snap.c
+++ b/drivers/md/dm-snap.c
@@ -2171,6 +2171,21 @@ static int origin_status(struct dm_target *ti, status_type_t type, char *result,
return 0;
+static int origin_merge(struct dm_target *ti, struct bvec_merge_data *bvm,
+ struct bio_vec *biovec, int max_size)
+ struct dm_dev *dev = ti->private;
+ struct request_queue *q = bdev_get_queue(dev->bdev);
+ if (!q->merge_bvec_fn)
+ return max_size;
+ bvm->bi_bdev = dev->bdev;
+ bvm->bi_sector = bvm->bi_sector;
+ return min(max_size, q->merge_bvec_fn(q, bvm, biovec));
static int origin_iterate_devices(struct dm_target *ti,
iterate_devices_callout_fn fn, void *data)
@@ -2188,6 +2203,7 @@ static struct target_type origin_target = {
.map = origin_map,
.resume = origin_resume,
.status = origin_status,
+ .merge = origin_merge,
.iterate_devices = origin_iterate_devices,