Calculating speed of harddrives (Part 3)

So, what about Random transfer ? As I mentioned earlier (Part 1), the speed of a harddrive depends on 2 factors : Average Latency and Average Seek. On an average SCSI harddrive, the Seek = 3 Milliseconds. But when you need to look for your data on a totally different place on the harddrive, both latency and seek need to be taken into account. So 6ms for each access to the HD in case of Random. 1 I/O takes 6ms, in 1 second, 1000/6 = 166 IO's/second. As I mentioned earlier, databases transfer small chunks of data lets say : 4KB. 166*4 = 664KB/s. you would need some 640 harddisks to reach the maximum speed of Ultra 4 SCSI.