RAM and Swap Disk Tips
RAM and Swap Disk
Whatever the operating platform, the processing demands of the Oracle
database must not exceed the real RAM memory of the server. All large
servers share RAM resources through a Virtual Memory (VM) scheme. Oracle
servers (Windows, UNIX, and OS-390) manage excessive RAM demands with a
special swap disk, and virtual memory exploits the fact that not every
executing task is constantly referencing its RAM memory region.
Since some RAM regions are accessed intermittently, vendors have developed
paging algorithms that shift these memory pages to the swap disk when they
are not in use.
The swap disk is a special disk area that provides for RAM sharing,
primarily by storing page frames of inactive disk programs. The
least-frequently-used (LRU) RAM pages are offloaded so that new
applications can simultaneously share the same memory. After inactive RAM
frames are paged-out to the disk, the operating system can utilize the
freed memory for other active tasks.
If the inactive program later resumes execution, the RAM frames are
paged-in from the swap disk.
This reloading of RAM pages from one memory area to another is called
swapping. Swapping is very time-consuming and degrades performance of the
The swap disk allows simultaneous RAM usage greater than the real amount of
RAM, but performance will be seriously compromised if the swap disk is used
for active programs. The impaired performance is due to the much slower
read times from the swap disk, compared to RAM. Disk access is measured in
milliseconds, or millionths of a second, while RAM access is measured in
nanoseconds, or billionths of a second.
Not all of this difference can be practically realized because of Oracle
overhead, but most experts say that Oracle RAM is about 2,500 times faster
than disk access.
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