IBM SG24-4576-00 manual NetWare Integration Guide

which represents the maximum server transactions per second or throughput rate.

After the peak throughput is reached, the curve begins to slope downward. As the number of users is increased, the caching engine of the operating system begins to break down. The reduction in the disk cache hit rate is caused by the increasing amount of data that each additional user requests the server to access. The ratio of requested data to the size of the network operating system disk cache increases to a point where the network operating system disk caching is no longer effective. Furthermore, the disk cache is often storing data that was requested to be written to disk by application users. This write caching, when employed, causes write-data to occupy disk cache memory space until it is finally written to the disk surface. Thus, slow disk write operations can compound the performance degradation of servers that support write caching because the amount of useable disk cache memory is reduced by write-data waiting for service by the slower disk system.

The curve continues to break down until it reaches the transaction rate sustainable by the disk subsystem. In this state, few I/O requests can be serviced directly from disk cache and most require direct disk access. At this point, the server has reached saturation.

The initial slope of the curve is dependent upon how quickly transactions can be processed by the server, which, in turn, depends primarily on how quickly the LAN adapter is able to transfer data.

The peak of the curve is the maximum server transaction rate that the particular server configuration is capable of sustaining for a specific transaction type. The maximum transaction rate is primarily dependent upon performance of the network adapter and disk subsystem combination. When the graph flattens out, performance of the disk subsystem has the greatest influence on overall server performance.

We can gain a better understanding of how to tune server hardware by studying the curve dynamics as we modify various hardware components. Changing the LAN adapter or the disk subsystem can alter the height or width of the graph.

For example, in Figure 141 on page 171, adding a faster network adapter will increase the initial slope of the graph and also provide additional throughput. Therefore, the total number of transactions that can be processed per second will be improved. Furthermore, response time seen by any individual application user will be reduced. However, the reduction in response time is a limited benefit. As more users are added, the effects of the improved network adapter will be offset by increased disk I/O and increased write-data, which reduces the disk cache hit rate and overall server performance.

170NetWare Integration Guide