The SHMVIRTSIZE parameter in the Informix ONCONFIG file specifies how much shared memory in KB to use for the "virtual portion" at system start. Informix stores various objects here, including the dictionary cache.

The most important factor affecting the size of virtual portion is the session pools. If a large number of users are very active, Informix adds memory to this area. This dynamic allocation of shared memory can affect performance considerably, so you should allocate a large initial segment to avoid dynamic increases.

Adjust the parameters SHMVIRTSIZE and SHMADD to the actual storage requirements of the system for the following reasons:

• The dynamic allocation of new shared memory segments is a time-consuming operation, which should be avoided in online operation for reasons of performance.
• Various operating systems have a limit on the available shared memory segments.
• For different operating systems, there are significant performance losses if more than two shared memory segments are used.

You can use the main screen of the database monitor to check for Virtual Portion in the Shared Memory display, which shows the value of SHMVIRTSIZE . Or you can execute the command onstat -g seg (as the user <sid>adm/informix or with transaction ST04), as in the following example:

Set the parameter SHMVIRTSIZE to a value which can be determined as follows in a production system:

Shared Memory (Informix)

SHMADD (Informix)

SHMTOTAL (Informix)