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Commit b1d55528 authored by Mikulas Patocka's avatar Mikulas Patocka Committed by Alasdair G Kergon
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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: default avatarMikulas Patocka <mpatocka@redhat.com>
Signed-off-by: default avatarAlasdair G Kergon <agk@redhat.com>
parent 4a0b4ddf

drivers/md/dm-snap.c

+16 −0
Original line number Original line 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;

	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,

static int origin_iterate_devices(struct dm_target *ti,

				  iterate_devices_callout_fn fn, void *data)

				  iterate_devices_callout_fn fn, void *data)

{

{
@@ -2188,6 +2203,7 @@ static struct target_type origin_target = { 
	.map     = origin_map,

	.map     = origin_map,

	.resume  = origin_resume,

	.resume  = origin_resume,

	.status  = origin_status,

	.status  = origin_status,

	.merge	 = origin_merge,

	.iterate_devices = origin_iterate_devices,

	.iterate_devices = origin_iterate_devices,

};

};