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Estimating Required Additional RAID Performance Capacity

The Avg. Disk Queue Length for Configuration 1 is 16, which exceeds the maximum recommended rating of 6 (3 disks * 2 outstanding requests each). Also, the maximum transfers per second are 139 ([126 + (4 * 73)] / 3) per disk, which exceeds the suggested workload that one disk can support. The combination of long queues and excessive numbers of transfers per second slow the Avg. Disk sec/Transfer response time to 0.121 second.

You want to limit each disk in the array to no more than two outstanding requests at a time, so you need a minimum of eight disks to remove the bottleneck. I recommend that you replace the three-disk RAID 5 array with a 10-disk RAID 5 array. Adding two more disks than the system requires gives you some room for possible surges in workload and room to accommodate future requirements. This configuration removes the disk bottleneck and provides 36GB of usable storage capacity.

Graph 2 shows how the average response times of the RAID array in Configuration 1 compare with those of the new configuration (Configuration 2). Graph 1 shows how the throughput levels of the RAID array for Configuration 1 compare with those of Configuration 2. Configuration 2 lowered the average response time from 13.9 seconds to 9.2 seconds, and improved the throughput from 3.8MBps to 4.9MBps at the 20-client level. The Avg. Disk Queue Length dropped from 16 to 12, and Avg. Disk sec/Transfer dropped from 0.119 second to 0.049 second. These results provide insight into the reason why the throughput and response time reported by the Bluecurve clients improved significantly. In addition, Performance Monitor reported that the RAID array provided greater than 7.34MBps of disk throughput while supporting a workload of 68 ([147 + (4 * 117)]/9) transfers per second per disk. This sizing solution provides improved performance with room to grow.

Graph 2

Graph 2: File Server Response Time (seconds) Performance Results for Configuration 1 (3 Disk RAID 5 Array) and Configuration 2 (10 Disk RAID Array)

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