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Determining How Many Disks to Add

How do you know how many disks you need to meet your performance requirements? The primary performance requirements for a RAID array are adequate throughput and response time. The workload that you place on the array and the amount of work that the RAID array can support (for example, transfers per second) influence both requirements.

To help you know what steps you need to take when adding storage capacity to NT, let's look at an example. Imagine that you have a server with a RAID 5 array composed of three 4GB Ultra Wide SCSI 7200rpm hard disks. Having historical information to work from when adding storage capacity is helpful, so imagine that you've stress-tested your NT file server using Bluecurve's bidirectional copy workload to simulate a file server.

From the Bluecurve stress test results shown in Graph 1, you learn that the maximum throughput that this configuration (Configuration 1) provides at the 20-user level is 3.8MB per second (MBps), with a response time of 13.9 seconds. When you review the corresponding Performance Monitor log to determine what's happening inside NT during the tests, I observed that the %Disk Time stays at 100 percent. As the Disk Transfers/sec increases against the disk array, the Avg. Disk Queue Length grows to almost 16, and the average RAID array response time (Avg. Disk sec/Transfer) increases to 0.121 second, which is slow. This information indicates that this RAID array is causing a bottleneck. Now that you know that a bottleneck is occurring, you can use this information to determine the best economical solution to remove the bottleneck and increase the usable disk capacity.

Graph 1

File Server Throughput (Bytes/sec) Performance Results for Configuration 1 (3 Disk RAID 5 Array) and Configuration 2 (10 Disk RAID Array)

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