Check IO scheduler the disk is using

cat /sys/block/<dev>/queue/scheduler

# cat /sys/block/sdam/queue/scheduler 
noop anticipatory deadline [cfq]
In the output above, cfq is the default io scheduler

Change io scheduler for a disk

echo ioscheduler > /sys/block/dev/queue/scheduler

In case of using multipath, you need to do all channels, for example:

# multipath -ll ddnlun2

ddnlun2 (360001ff014e2181202000800003a381b) dm-12 IBM,DCS9550
\_ round-robin 0 [prio=2][active]
 \_ 4:0:2:2  sdam 66:96  [active][ready]
 \_ 3:0:0:2  sdc  8:32   [active][ready]

# echo deadline > /sys/block/sdam/queue/scheduler
# echo deadline > /sys/block/sdc/queue/scheduler