IQ_SYSTEM_MSG is a system dbspace that points to the file path of the database IQ message log file.
IQ_SYSTEM_MSG is not considered a store because it does not store any data. It has one file per multiplex node. By default, the physical file name for the message file on a simplex server or a coordinator of a multiplex is <dbname>.iqmsg. The physical file name for the IQ message file on a secondary node in a multiplex is <servername>.iqmsg.
Because it is not an IQ store dbspace, ALTER commands such as READONLY and OFFLINE do not apply to IQ_SYSTEM_MSG.