thanks God we use sata now, then you have no worry about optimizing the disk, because on sata disk the DMA is on by default and always on.

and the speed is depend to the drivers of the disk, so maybe you have to choose the right hardware for your server.

below is my tests, the best is from the test on intel xeon with intel motherboard (i dont know yet the other spec :)

[root@server1 admin]# /sbin/hdparm -tT /dev/sda

/dev/sda:
 Timing cached reads:   25284 MB in  1.99 seconds = 12674.42 MB/sec
 Timing buffered disk reads:  230 MB in  3.00 seconds =  76.60 MB/sec

Source: widyamedia.net