Each Microsoft Exchange Server database transaction is written to the transaction log before being written to the database. At any given time there will be an "inactive" part of the transaction log consisting of transactions that have already been committed to the database, and an "active" part consisting of transactions still to be committed. During an online, full backup or an online, incremental backup, the "inactive" part of the transaction log is deleted.
The transaction log comprises a set of files, each of which is exactly 5MB in size. New transactions are written to the file Edb.log. As the Edb.log file becomes full (that is, after 5MB of transactions have been written to it), it is renamed and a new Edb.log file is created. The renamed log file is stored in the same subdirectory as the Edb.log file but is not committed to the database. Log files are named in a sequential order using hexadecimal numbers (for example, Edb00009.log, Edb0000A.log, and so on).
Note When circular logging is enabled, previous .log files are not maintained and therefore are not purged by backup operations. Incremental and differential online backups are not permitted when circular logging is enabled.
When the service is shut down normally (that is, with no errors), transactions in log files are committed to the respective .edb file. For example, when the information store service is shut down normally, transactions that existed in log files and not in Priv.edb or Pub.edb files are committed to the .edb files. Note that .log files should not be manually purged while services are running. In general, it is best to purge logs using the backup process.