Figure 22 – Windows 2000 Multiple IDE disk IOs per second

Figure 23 shows unbuffered random KAPS for two disks along with a two disk RAID0 stripe set. Unlike Figure 22, the request depths shown are the total number of IOs outstanding instead of the number per drive. Thus, a four-deep request depth in Figure 23 is equal to two-deep per drive for two disks, or one-deep per drive for four disks. The point to note in Figure 23 is that adding additional disks to a stripe set does not result in linear gains. This is because, unlike the case in Figure 22, random requests aren’t being sent to each disk individually but rather to a single logical volume. Thus, parallelism isn’t guaranteed. When sending two random requests to a logical volume, there is a 50% chance that both requests will go to the same drive rather than separate drives.