The ARCH process is used to copy the contents of an online log file to another location, typically a disk file, when that log file becomes full. Oracle uses the online log files in a “round robin” fashion—that is, when all available online log files become full, the first file is reused. The mode of operation whereby the contents of each file are saved prior to reuse is called archivelog mode, and is controlled by the ARCHIVELOG parameter in the ALTER DATABASE statement. The ARCH process runs only when the instance is running in archivelog mode. For an additional discussion of archivelog mode, see Chapter 4.