SWAP Disk Uses

In order to provide for the sharing of RAM, a special area of disk called a swap disk is required, and the primary purpose of the swap disk is to hold page frames from in active programs on disk. The purpose of the swap disk is to offload the least-frequently-used (LRU) RAM page frames so that many applications can concurrently share the same memory.

Once RAM pages from inactive programs are written to disk (a page-out), the operating system can make the freed RAM memory available for another active task. Later, when the inactive program resumes execution, the RAM pages are re-loaded from the swap disk into RAM (a page-in). This reloading of RAM pages is called swapping, and swapping is very time-consuming and degrades the performance of the target program.

While having the swap disk ensures concurrent RAM usage above the real amount of RAM, optimal performance requires that the swap disk is never used for active programs. This is because reading RAM pages off of the swap disk is about 14,000 times slower than reading the memory pages from directly from RAM. As we know, disk access is measured in milliseconds, or thousandths of the second, while RAM access is access to in nanoseconds, or billionths of a second

In a VM architecture, the OS will write Ram to the swap disk, even thought the real RAM has not been exceeded. This is done in anticipation of a RAM shortage, and if a real RAM shortage occurs, the LRU RAM frames are already on the swap disk.

For an Oracle server, the goal is to keep all of the RAM memory demands of the database and database connections beneath the amount of physical RAM memory. In an Oracle environment, we can accurately control the amount of RAM memory that is used by the database instance System Global Area (SGA). This is because the Oracle database administrator can issue alter system command to change the RAM memory areas, and can grow and shrink the RAM memory areas on as needed basis.