Chapter 6. Swap Space

In some cases it is good for the swap partition to be used. For example, long running processes often access only a subset of the page frames they obtained. This means that the swap partition can safely be used even if memory is available because system memory could be better served for disk cache to improve overall system performance. In fact, in the 2.6 kernel used in Red Hat Enterprise Linux 4 and 5, you can define a threshold when processes should be swapped out in favor of I/O caching. This can be tuned with the /proc/sys/vm/swappiness kernel parameter. The default value of /proc/sys/vm/swappiness is 60 which means that applications and programs that have not done a lot lately can be swapped out. Higher values will provide more I/O cache and lower values will wait longer to swap out idle applications. Swappiness percentage may be tuned using:

echo 10 > /proc/sys/vm/swappiness

# echo 10 > /proc/sys/vm/swappiness

or,

echo vm.swappiness=10 >> /etc/sysctl.conf

# echo vm.swappiness=10 >> /etc/sysctl.conf

Depending on your system profile you may see that swap usage slowly increases with the time the system is up. To display swap usage you can run the free(1) command or you can check the /proc/meminfo file. When the system uses swap space it will sometimes not decrease afterward. This saves I/O if memory is needed and pages do not have to be swapped out again when the pages are already in the swap space. However, if swap usage gets close to 80% - 100% (your threshold may be lower if you use a large swap space), then a closer look should be taken at the system, see also Section 6.2, “Checking Swap Space Size and Usage”. Depending on the size of your swap space, you may want to check swap activity with vmstat or sar if swap allocation is lower than 80%. But these numbers really depend on the size of the swap space. The vmstat or sar command output the number of pages swapped. This output field is an important metric. This number should be low or zero as constant page swapping should be avoided at all costs.

Swap Size Recommendations According to Oracle9i Installation Guide Release 2 a minimum of 512MB of RAM is required to install Oracle9i Server. According to Oracle Database Installation Guide 10g Release 2 at least 1024MB of RAM is required for 10g R2.

For 10g R2, Oracle gives the following swap space requirement: