Commit b1d55528 authored Aug 12, 2010 by Mikulas Patocka Committed by Alasdair G Kergon Aug 12, 2010

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>

parent 4a0b4ddf

drivers/md/dm-snap.c

+16 −0

Original line number	Diff line number	Diff line
		@@ -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,
		};

Admin message