How OnLine Manages Shared Memory

Shared memory is an area of the system's memory that is available to all OnLine users. Much of the shared memory is used by buffers. Using shared memory for these buffers enables OnLine to pool its memory resources for multiple servers rather than set aside individual buffer areas for each process.

These buffers are used to buffer pages to and from the physical disk storage on the system. Only the most recently used pages are kept in the shared memory buffers. Thus, if a needed disk page is in the buffers, it will be the latest version and can be accessed quickly. As the buffers begin to fill or as the system detects the need to flush the buffers, buffered pages are written back to disk. When the system needs to read data from the disk, it looks for the page in the buffers first. If it's there, the page is accessed from the buffer, saving on time-consuming disk I/O. If the needed page is not in the buffers, the system chooses a free buffer, if there is one. If there is no free buffer, the system forces the oldest page (the least recently used, or LRU) to flush to disk freeing up that page.

OnLine shared memory segments are allocated by database instance. Each instance will maintain its own shared memory, and each instance will require adequate resources to be allocated for its use. The DBA has control over many parameters that determine the size and division of the shared memory.

The tbmonitor utility allows for the sizing of the shared memory. As different parameters are changed, the program will indicate how much shared memory is needed.

If your UNIX kernel has been configured with adequate shared memory resources, you can alter the amount of shared memory used by changing the parameters in tbmonitor. Always add a little "fluff factor" to the suggested shared memory. For instance, if tbmonitor indicates that 13 megabytes shared memory is needed, make sure that UNIX is ready to give you an extra megabyte or so. This will allow you to have some flexibility for later tuning of the OnLine parameters without having to rebuild the kernel.