We had a server performing pretty poorly and we went to investigate.
It seemed the disk speed was ~ 2.15 MB / sec, which isn't good.
First check if DMA is enabled on the disk with following command:
# hdparm -d /dev/hda
You should get the following output:
using_dma = 0 (off)
Now let's enable DMA for the disk:
# hdparm -d 1 /dev/hda
You will see that DMA is enabled now and the disk speeds are much faster!
Remark: this has been tested with older ATA disks and is no longer relevant for newer harddisks.