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Disk Storage Capacity vs. Disk Performance Capacity

In the example in this article, you learned how to determine the number of disks you need to add to a RAID array to remove a disk bottleneck and to provide the necessary storage capacity. This example provides 36GB of usable disk storage capacity. So why did I suggest that you create a RAID array using ten 4GB disks instead of five 9GB disks to provide 36GB of usable storage capacity? The answer has to do with the supported disk workload. Just because disk capacity increases from 4GB to 9GB, the workload each disk can support doesn't increase if the disks in the RAID array are from the same family (such as Ultra Wide SCSI 7200rpm). Regardless of the disk storage capacity, each 7200rpm disk can support only about 100 transfers per second. Thus, if you use five 9GB disks instead of ten 4GB disks, you meet the storage capacity goal of 36GB, but the RAID array is still a bottleneck. You can also use nineteen 2GB disks to provide even better performance, but this solution is economically prohibitive.

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