18 disks thats a lot p.rn

btw, you might look at I/O schedulers that you are using for your drives.
if you want CFQ(this is default) just:
echo "cfq" > /sys/block/sdX/queue/scheduler (where sdX is your drive...)
other schedulers are deadline(which I am using) and noop.

other things you might want to look at are these:
/proc/sys/vm/dirty_expire_centisecs
/proc/sys/vm/dirty_ratio
/proc/sys/vm/dirty_background_ratio
/proc/sys/vm/dirty_bytes
/proc/sys/vm/dirty_background_bytes

just google them around