Sizing your DASD log streams

If you are capacity planning for new applications, you must calculate your space requirements.

The formulae provided help you to calculate values for LOWOFFLOAD and STG_SIZE.

You must base your calculations on the journaling requirements of your applications. These requirements provide the starting point for the following formulae.

You can calculate LOWOFFLOAD for DFHLOG using the following formula:

LOWOFFLOAD = (trandur * 90) / (akpintvl + trandur) + 10  (where RETPD=0 specified)

LOWOFFLOAD = (trandur * 90) / (akpintvl + trandur)    (where RETPD=dddd specified)

where:

akpintvl is the interval between activity key points. Calculate it using the following formula:
```
akpintvl = AKPFREQ / ( (N1 * R1) + (N2 * R2) + (Nn * Rn) )
```
where:
- N1, N2 .... Nn is the transaction rate for each transaction (transactions per second).
- R1, R2 .... Rn is the number of log records written by each transaction.
trandur is the execution time (between sync points) of the longest-running transaction that runs as part of the normal workload.
If this duration is longer than the akpintvl value, you have these choices:
- Increase the value of AKPFREQ, thus increasing the value of akpintvl, provided that this value does not result in an unacceptably large staging data set size.
- Change the application logic to cause more frequent sync points.
- Calculate a staging data set size based on a shorter transaction duration, and accept that offloading to auxiliary storage occurs when the long-running transaction is used.

You can calculate STG_SIZE for DFHLOG using the following formula:

Staging DS size    =  (AKP duration)  *  No. of log writes per second
for system log (no. of 4k blocks)
where AKP duration = (CICS TS 390 AKPFREQ)/(No. buffers per second)

You can take the values for the number of log writes per second and buffer puts per second from your statistics. The value for log writes per second must not exceed 30.