Shared memory

You must configure adequate shared-memory resources for the database server in your operating system. Insufficient shared memory can adversely affect performance.

The database server threads and processes require shared memory to share data by sharing access to segments of memory.

The shared memory that Informix® uses can be divided into the following parts, each of which has one or more shared memory segments:
  • Resident portion
  • Virtual portion
  • Message portion
  • Buffer pool portion

The resident and message portions are static; you must allocate sufficient memory for them before you bring the database server into online mode. (Typically, you must reboot the operating system to reconfigure shared memory.) The virtual portion of shared memory for the database server grows dynamically, but you must still include an adequate initial amount for this portion in your allocation of operating-system shared memory.

The amount of space that is required is the total that all portions of database server shared memory need. You specify the total amount of shared memory with the SHMTOTAL configuration parameter.

The LOCKS configuration parameter specifies the initial size of the lock table. If the number of locks that sessions allocate exceeds the value of LOCKS, the database server dynamically increases the size of the lock table. If you expect the lock table to grow dynamically, set SHMTOTAL to 0. When SHMTOTAL is 0, there is no limit on total memory (including shared memory) allocation.


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