actual disk iops and write merging

/dev/sdb has LVM and exposes a single logical volume, dm-0.

While writing to dm-0, /proc/diskstats reports:

# of writes completed
sdb 29646717
dm-0 131315099

# of writes merged:
sdb 101668283
dm-0 0

That means that /dev/sdb statistics report that most of the writes have been merged, so it report much lower total writes, while the lvm device does not. Which counters are more indicative of the actual write IOPS detected by the lower layer (e.g. AWS provisioned IOPS?)