You must configure adequate shared-memory resources for the database server in your operating system. Insufficient shared memory can adversely affect performance. When the operating system allocates a block of shared memory, that block is called a segment. When the database server attaches all or part of a shared-memory segment, it is called a portion.
The database server uses the following shared-memory portions. Each portion makes a separate contribution to the total amount of shared memory that the database server requires:
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 following sections provide guidelines for estimating the size of each shared-memory portion for the database server so that you can allocate adequate space in the operating system. The amount of space required is the total that all three portions of database server shared memory need, which the SHMTOTAL configuration parameter specifies.
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