Controlling Audit File Size

How the Accessroutines Calculates the Tape Capacity

The Accessroutines uses the values for tape length and tape density to calculate the tape capacity. You can specify these values in the DASDL description, but doing so is optional.

An audit file switch is caused when the physical specifications for the audit file (either those specified by the user or the defaults calculated by DASDL) are reached or before the tape capacity is exceeded, whichever comes first.

How the Accessroutines calculates the tape capacity from the values for tape length and tape density depends on whether the values are specified in the DASDL description and, if the tape length is not specified in the DASDL description, on whether REAPPLYCOMPLETED is set, as follows:

  • The tape length is specified in the DASDL description.

    The Accessroutines chooses either of the following values:

    • If you specify the tape length for only one of the primary audit file or secondary audit tapes, the Accessroutines uses that value for both audit tapes.

    • If you specify different tape length values, one for the primary and one for the secondary audit file, the Accessroutines uses the smaller of the two values.

    The Accessroutines calculates the tape length as follows:

    TAPE LENGTH IN INCHES = 0.98 * 12 * TAPE LENGTH IN FEET

  • The tape length is not specified in the DASDL description and the REAPPLYCOMPLETED option is set.

    The Accessroutines uses 2400 feet for all densities. The Accessroutines calculates the tape length as follows: 

    TAPE LENGTH IN INCHES = 0.98 * 12 * 2400

    In the previous example, the Accessroutines causes an audit file switch before tape capacity is exceeded, regardless of the maximum size of the audit file resulting from specifications of AREASIZE, AREAS, or BLOCKSIZE.

  • The tape length is not specified in the DASDL description and the REAPPLYCOMPLETED option is not set.

The Accessroutines does not calculate the tape capacity. To ensure that an audit file fits on one reel of tape, you can specify values for AREAS and AREASIZE as shown in the examples under “Controlling Audit File Size” earlier in this section.

Efficiency Considerations

Efficiency Considerations for Audit Trail Options lists the efficiency measures you should consider when you designate options in your audit trail specification.

Table 5. Efficiency Considerations for Audit Trail Options

Option

Description

BLOCKSIZE

In general, increasing the audit trail block size decreases the number of I/O operations performed and improves database performance. However, a large audit trail block size also increases the amount of memory used for audit buffers.

UPDATE EOF

This attribute controls an important trade-off in database performance. Small values for the UPDATE EOF option reduce the number of disk read operations needed to locate the end of the audit trail during recovery. However, more write operations are performed to maintain the end-of-file pointer in block 0 (zero) during normal operation of the database.


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