Compaq ProLiant 1000 I/O Performance Tuning of Compaq Servers - Page 14

I/O Performance Tuning of Compaq Servers 14 In a FC-AL, all data requests must traverse the entire loop to reach the server. When a hub is used to connect multiple FC Arrays, passing data from one chassis to another requires a path through the hub. In other words, for every FC chassis data must travel between the hub and the chassis twice. In a six Fibre Channel Array system with 10 km between the server and hub, and an additional 10 km between the hub and each array chassis, the data must travel 140 km to complete the loop. Even when data is moving at the speed of light, 140 km introduces measurable delay (.467 msec for 140 km). P When deploying Fibre Channel storage over extremely large distances, the delays introduced can have an effect on performance. Fibre Channel Array Storage Arrays Just as with PCI, Compaq employs dual, independent SCSI busses in each Fibre Channel Array. The concept of balanced SCSI busses applies within each Fibre Channel Storage Array chassis. Since there are two independent busses, making certain that the I/O load is evenly distributed can provide higher sustained throughput to the Fibre Channel interface. When populating the Fibre Array with drives, you should take care to split the I/O load between the two busses. When drives of similar performance are used, balancing the SCSI busses is as simple as dividing the number of drives on each bus evenly. However, drives should not be divided evenly if dividing the drives between the SCSI busses will require mixing of SCSI revisions or placing a heavier load on one bus. Compaq storage on Fibre Channel is the Fibre Channel Array. These arrays consist of a chassis with a Fibre Channel adapter and bays for eight 1-1/2 inch drives or twelve 1 inch drives. There are two SCSI busses in each array; one on the left and the other on the right. Care must be taken when plugging drives into the array to insure the drives are distributed evenly over the two busses or contention could reduce the array's performance. Placing two drives on the same bus can reduce the performance as much as 5% from placing one drive on each bus. Placing four drives on one bus can degrade performance by as much as 7%. Six drives on one bus can yield a 20% lower performance than a balanced distribution. P When deploying disk drives in Fibre Channel storage, distribute them evenly between the busses in the array. Drive Technology Since the physical hard drive must retrieve and write any and all data, delays caused by the drive are propagated throughout the I/O chain. So, it is important to understand what factors affect drive performance and what can be done to minimize the effect of the unavoidable delays. The hard drive is the only device in the I/O chain that contains moving parts and, therefore, is subject to physical delay. Because the drive head must move across the spinning platter, delays are introduced by locating the data and positioning the head on the platter. When characterizing the delays associated with hard drives, there are three primary parameters: • Rotational Latency • Seek Time • Access Time ECG044.0399