Most likely you're running of disk IO bandwidth (don't confuse with occupied disk space!) due to extensive downloading by end-users. In order to check that, we suggest you to install an utility named 'atop' ('yum install atop' in the case of CentOS), then run it for as least 10 seconds - now you'll see that some of your disks are marked with red.
To avoid this issue, you have to do the following:
1. Don't use the following for storing end-users files: Software RAID, Hardware RAID, LVM. Instead of that, we suggest to use JBOD configuration (just attached disks) with possibly using mhddfs for merging them into one partition.
2. Install a new extra file server, and move some of your popular files to it