The archiver processes (ARCn) exist only when the database is in ARCHIVELOG mode and automatic archiving is enabled, in which case ARCn automatically archives online redo log files. The log writer process (LGWR) cannot reuse and overwrite an online redo log group until it has been archived.
The database starts multiple ARCn processes as needed to ensure that the archiving of filled online redo logs does not fall behind. Possible processes include ARC0-ARC9 and ARCa-ARCt (31 possible destinations).
The LOG_ARCHIVE_MAX_PROCESSES initialization parameter specifies the number of ARCn processes that the database initially invokes. If you anticipate a heavy workload for archiving, such as during bulk loading of data, you can increase the maximum number of ARCn processes. There can also be multiple archive log destinations. Oracle recommends at least one ARCn process for each destination.
ARCn can run as a thread or as an operating system process.