System V shared memory allows the creation of a segment by a process. Cooperating processes can attach to the memory segment (subject to access permissions on the segment) and gain access to the data contained in the segment. This capability is implemented as a loadable module. Entries in the /etc/system file must contain the shmsys: prefix.
A special kind of shared memory known as intimate shared memory (ISM) is used by DBMS vendors to maximize performance. When a shared memory segment is made into an ISM segment, the memory for the segment is locked. This feature enables a faster I/O path to be followed and improves memory usage. A number of kernel resources describing the segment are then shared between all processes that attach to the segment in ISM mode.