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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: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>
hifive-unleashed-5.1
Mikulas Patocka 2010-08-12 04:14:02 +01:00 committed by Alasdair G Kergon
parent 4a0b4ddf26
commit b1d5552838
1 changed files with 16 additions and 0 deletions
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16
drivers/md/dm-snap.c
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@ -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,
};