Controlling Auditing Costs

Because auditing consumes system resources, you must control the degree of detail that is recorded. When you decide what to audit, consider the following costs of auditing:

• Cost of increased processing time

• Cost of analysis of audit data

• Cost of storage of audit data

Cost of Increased Processing Time of Audit Data

The cost of increased processing time is the least significant of the costs of auditing. The first reason is that auditing generally does not occur during computation-intensive tasks, such as image processing, complex calculations, and so forth. The other reason is that the cost for single-user systems is usually small enough to ignore.

Cost of Analysis of Audit Data

The cost of analysis is roughly proportional to the amount of audit data that is collected. The cost of analysis includes the time that is required to merge and review audit records. Cost also includes the time that is required to archive the records and keep the records in a safe place.

The fewer records that you generate, the less time that is required to analyze the audit trail. Upcoming sections, Cost of Storage of Audit Data and Auditing Efficiently, describe ways to audit efficiently. You can reduce the amount of data that you collect, while still providing enough coverage to achieve your site's security goals.

Cost of Storage of Audit Data

Storage cost is the most significant cost of auditing. The amount of audit data depends on the following:

• Number of users

• Number of machines

• Amount of use

• Degree of security that is required

Because these factors vary from site to site, no formula can determine in advance the amount of disk space to set aside for audit data storage.

Full auditing, that is, with the all flag, fills up disks quickly. Even a simple task such as compiling a program could generate a large audit file. A program of modest size could generate thousands of audit records in less than a minute. For example, the audit trail of a program of five files and 5000 lines would occupy many megabytes of disk space. You judiciously use the preselection features to reduce the volume of records that are generated. For example, by omitting the fr class, you can reduce the audit volume by more than two-thirds. Efficient audit file management is also important. After the audit records are created, file management reduces the amount of storage that is required.

Before you configure auditing, you should understand the audit flags. You should understand the types of events that the flags audit. Develop a philosophy of auditing for your site that is based on sensible measures. Such measures include the amount of security that your site requires, and the types of users that you administer.