How can you tell if Squid is performing well? Does Squid have enough memory, bandwidth, and disk space? When the Internet seems slow, is it Squid's fault or a problem somewhere else? Is the operating system giving enough resources to Squid? Is someone trying to abuse or hack into my proxy? You can find the answers to these, and many more, questions in this chapter. Squid provides information about itself in three different ways: cache.log messages, the cache manager, and an SNMP MIB.
Squid writes various messages to cache.log as it runs. Most of these are abnormal events of one sort or another. Unfortunately, Squid isn't always smart enough to differentiate serious problems from those that can be safely ignored. Even so, cache.log is a good place to start when investigating a Squid problem.
The cache manager and SNMP interfaces allow you to query Squid for a variety of data. The cache manager, which has its own shortcomings, probably provides the most information in current versions of Squid. It has a TCP socket-based interface and tries to generate output suitable for both human and computer processing. The bulk of this chapter is devoted to explaining all the information available from the cache manager.
Squid supports SNMP as well. Unfortunately, the data available through SNMP is only a subset of the cache-manager information. Additionally, the Squid MIB has not evolved much over the years; it's essentially unchanged since its first incarnation. I'll explain how to make Squid process SNMP queries and describe all objects in the current MIB.