Well your problem is that you have all the data stored on the same physical disks so you are causing all sorts of contention on the disks. If you are using 8 drives in a RAID 10, then you actually only get to use 4 as the other 4 are a mirror of the first 4.
What are you using to mirror from the first 8 disks to the second 8 disks? Are you doing a clone within the array?
What kind of array is it? What kind of disks are in it? How fast are they?
You should break up the LUNs so that they are on their own disks instead of a single RAID 10. Do you actually need RAID 10? What is the data change rate within the database? What percentage of calls to the database are reads and what percentage of calls are writes?