Further, the analyst must resist the temptation to tune what is measurable rather than what is important. If no system-provided measurement corresponds to the desired improvement, one must be devised. Unless these priorities are set in advance, and understood by all concerned, the analyst cannot make trade-off decisions without incessant consultation and is apt to be surprised by the reaction of users or management to aspects of performance that have been ignored. If the support and use of the system crosses organizational boundaries, a written service-level agreement between the providers and the users may be needed to ensure that there is a clear common understanding of the performance objectives and priorities. In general, the performance of a given workload is determined by the availability and speed of one or two critical system resources.
Systems have both real and logical resources.
Critical real resources are generally easier to identify, since more system performance tools are available to assess the utilization of real resources. Disk space Network access Logical resources are less readily identified. Logical resources are generally programming abstractions that partition real resources. The partitioning is done to share and manage the real resource. It is important to be aware of logical resources as well as real resources.