Calculating average log record size

One way to determine how much log volume you need is to consider the average size of a log record.

About this task

As a general estimate, you can start with 200 bytes as the average size. To increase the accuracy of this estimate, get the real size of the log records that are written.

Procedure

To calculate the average size of log records that are written:

  1. Collect the following values from the statistics report:
    LOG RECORDS CREATED
    The number of log records created ( C )
    LOG CI CREATED
    The number of control intervals created in the active log counter ( D )
  2. Apply the following formula to the log statistics values: 
     D  × 4 KB / C  = avg size of log record

Example

For example, you can gather statistics for logging activities from the IBM® OMEGAMON® for Db2 Performance Expert on z/OS® statistics report. A non-zero value for  A  in the following example indicates that the output buffer is too small. Ensure that the size you choose is backed up by real storage. A non-zero value for  B  is an indicator that the output buffer is too large for the amount of available real storage.

Figure 1. Log statistics in the IBM OMEGAMON for Db2 Performance Expert on z/OS statistics report
LOG ACTIVITY                 QUANTITY  /SECOND  /THREAD  /COMMIT
---------------------------  --------  -------  -------  -------
READS SATISFIED-OUTPUT BUFF      0.00     0.00      N/C     0.00
READS SATISFIED-OUTP.BUF(%)       N/C
READS SATISFIED-ACTIVE LOG       0.00     0.00      N/C     0.00
READS SATISFIED-ACTV.LOG(%)       N/C
READS SATISFIED-ARCHIVE LOG      0.00     0.00      N/C     0.00
READS SATISFIED-ARCH.LOG(%)       N/C

TAPE VOLUME CONTENTION WAIT      0.00     0.00      N/C     0.00
READ DELAYED-UNAVAIL.RESOUR      0.00     0.00      N/C     0.00
ARCHIVE LOG READ ALLOCATION      0.00     0.00      N/C     0.00
ARCHIVE LOG WRITE ALLOCAT.       0.00     0.00      N/C     0.00
CONTR.INTERV.OFFLOADED-ARCH      0.00     0.00      N/C     0.00
LOOK-AHEAD MOUNT ATTEMPTED       0.00     0.00      N/C     0.00
LOOK-AHEAD MOUNT SUCCESSFUL      0.00     0.00      N/C     0.00

UNAVAILABLE OUTPUT LOG BUFF  A   0.00     0.00      N/C     0.00
OUTPUT LOG BUFFER PAGED IN  B    0.00     0.00      N/C     0.00

LOG RECORDS CREATED  C        9456.6K    10.5K      N/C    16.41
LOG CI CREATED  D              277.3K   308.18      N/C     0.48
LOG WRITE I/O REQ (LOG1&2)     758.3K   842.70      N/C     1.32
LOG CI WRITTEN (LOG1&2)        976.8K  1085.48      N/C     1.70
LOG RATE FOR 1 LOG (MB)           N/A     2.12      N/A      N/A