The logging facility implements a circular “write-ahead” transaction log file for the management of transactions within the installation. It ensures that log records are written in a way that makes them accessible to the recovery and archiver processes.

The logging facility maintains in-memory log buffers. For every in-memory log buffer, there is a corresponding disk log file. Memory log buffers and disk log file blocks are the same length. The number of log buffers is configurable, set according to the performance requirements of the system. The logging system manages multiple asynchronous writes of log buffers to the log file.

The number of disk log file blocks corresponds to the size of the log file, and is specified when the log file is created. In a properly tuned system, most log file buffers are completely full of log records when they are written to the log file. However, as all log records associated with a transaction must be forced to the log file at certain times, principally at end transaction time, a small percentage of log file blocks may contain unused space. Server group commit logic is designed to minimize the frequency of log force operations and increase log file space use.

All servers in the Ingres installation, as well as the recovery and archiver processes, share the logging facility. Log records written by different servers, or written by several threads in the same server, can be combined with log records written by other servers or threads. The Logging Facility performs the following actions: