Transaction performance - VSAM

A transaction is a business unit of work produced by an online or batch interaction with an end user or a department with the system. It can be a CICS, TSO, a WEB, an APPC, distributed DB2, or even a batch interaction. If your MVS is in Workload Manager (WLM) goal mode, all these transactions are monitored and accounted for by z/OS.

To characterize the performance of a transaction, we need to understand its different response time components.This shows general response time components, where:

  • Tr = Ts + Tw
  • Ts is Service Time, Tw is Waiting Time (also called queue time), and Tr is Response Time

    Transaction performance

  • Exploding the above formula, we have:
    Ts = Ts(CPU) + Ts(IO) + Ts(TP)
    Tw = Tw(CPU) + Tw(IO) + Tw(TP) + Tw(Storage) + Tw(Other)
  • ETR = Ne / T
    ETR is the External Throughput Rate, Ne is the number of ended transactions, T is the elapsed time.
  • There is also one formula relating the Tr with ETR, derived by Little’s law:
    ETR = N / (Tt + Tr)
  • In this formula, N is the average number of users sending transactions (logged on), and Tt is the average thinking time of these users. Following are some considerations regarding this formula:

    – The variables that more intensively affect the ETR are N and Tt due to their usual numeric values. Therefore, you should never accept an SLA specifying ETR, because the only variable that the IS department can directly control is Tr.

    – However, experiences show that when Tr is in a sub-second value, the value of Tt drops dramatically. This fact has to do with the human behavior in front of a terminal. If the machine responds fast (in a sub-second value), we also respond fast.

Transaction response time is the best way to characterize the performance of a transaction. Here, the target is to reduce its value and consequently to increase the ETR figures (when the value is below one second). Remember that in RMF reports the Ts(TP) and Tw(TP) are not included in the response time pictured.

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