The two background processes MMON and MMNL are deeply involved with ASH activity. MMON, short for Manageability Monitor, wakes up every few seconds (but like DBWR and LGWR, it can be woken up by another process) and is involved in writing ASH buffers to the AWR tables. MMNL, short for Management Monitor Light, is responsible for gathering active session details once every second (default). It gathers operating system information (for example, from /proc/stat) and is also involved with writing ASH buffers to the AWR tables.

One way of visualizing the ASH buffers is to think of a ring structure. This model helps you understand how new ASH records are written into memory, how old ASH records are overwritten, how ASH buffers are referenced via the v$active_session_history view, and how ASH records are written to the AWR tables.

By default, every second, the MMNL background process wakes up and gathers active session information and writes the information in the ASH buffers, starting from where it last finished writing. This means the oldest ASH buffer resides just before-just ahead of-the next MMNL ASH buffer write. Since the ASH buffers form a ring structure, unless the records are archived, they will eventually be overwritten. Oracle will not allow this, so once an hour, or when the ASH ring structure is getting full, the MMNL and MMON background processes are woken and are involved in archiving ASH buffers to the AWR tables. Picture this archiving going clockwise, or forward in time.