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Database Essentials
Administrative Planning : Memory usage

Memory usage

The primary function of system memory is to reduce disk I/O. Memory speed is orders of magnitude faster than disk speed. From a performance perspective, reading and writing to memory is much more efficient than reading and writing to disk. Because memory is not a durable storage medium, long-term storage on memory is not an option. There are RAM disks that do provide durable data storage, but they are cost prohibitive for most uses.
Maximizing memory management includes:
*Allocating memory for the right tasks
*Having enough memory to support your needs
Systems manage memory with paging. There are two types of paging:
*Physical paging— Identifies when information is needed in memory; information is retrieved from temporary storage on disk (paging space)
*Virtual paging — Occurs when information is moved from one place in memory to another
Both kinds of paging occur on all systems. Under normal circumstances, virtual paging does not degrade system performance to any significant degree. However, too much physical paging can quickly lead to poor performance. Paging varies by hardware platform, operating system, and system configuration. Because virtual paging is fairly inexpensive, a significant amount can be done with no adverse effect on performance. Physical paging will usually be high immediately after booting the system, and it should level off at a much lower rate than virtual paging. Most systems can sustain logical paging levels of thousands of page requests per second with no adverse effect on performance. Physical paging levels in the thousands of requests is too high in most cases. Physical paging should level into the hundreds of page requests per second on most systems.
If physical paging continues at a high rate, then you must either adjust memory allocation or install more memory. It is important to remember that these numbers are only guidelines because your system might be able to handle significantly more requests in both logical and physical page requests with no effect on performance.
* Estimating memory requirements
* Optimizing memory usage