Oracle Tips by Burleson

Disk I/O Tuning

Chapter 1: Disk Capacity - The Two-Edged Sword

So to achieve the same IO capacity as we had with our 112-9 gigabyte disks we would need to buy 2.7 (952/348) times the needed capacity of 180 gigabyte drives even with their superior access times and MB/s transfer rates. The values in table 1-1 reflect the performance values when that drive capacity was the maximum available. Of course as we saw in Figure 1.7, if we replace the old 9 gig drives with the new 40 gig (36 gig formatted) drives with the sustained transfer rate of 58 MB/s we get 28*58 or a total transfer rate of 1624 MB/s which would require 28 of either the new 73 or 180 gig drives in order to match the IO rate from the 28-40 gig drives.

So if we used the new 40 gig drives, we could expect better IO transfer rates of up to 70 percent based on IO rate, while if we bought the 180 gig drives based on storage capacity alone we would see a decrease in IO capacity of over 270%. This is why IO capacity should be the driving factor in disk drive purchase, not storage volume per disk.