A feature of Microsoft Exchange Server whereby transaction logs can be overwritten when full. Circular logging lowers disk space usage but reduces the chances of successfully recovering from a system crash.
Exchange Server databases, such as the directory database and information store, maintain special log files called transaction log files. These log files improve the performance and fault tolerance of the databases, and help track and maintain changes made to them. Transactions are immediately written both to the log files and to memory, and only afterward to the database files. Transaction logs are normally kept on a different drive from the database files to ensure fault tolerance in case of a disaster that causes data loss, such as a crashed disk or a power failure.
When circular logging is enabled, only a few transaction log files are maintained, and these are overwritten when they become full. This prevents log files from continually building up, which saves disk space. However, circular logging has the disadvantage of allowing you to perform only full backups, rather than incremental or differential ones, because you can restore information only up to the last full backup.
Do not use circular logging if data recoverability is of high importance to your mail system, which is almost always the case with e-mail. Circular logging is enabled by default. You should always disable it and ensure that you have enough free disk space to hold the transaction files. The only reasons you might want to enable circular logging would be if you run low on disk space or if your server is being used for noncritical data only, such as a public news server.