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Chapter 6.
Swap Space
6.1. General
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
or,
# 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
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.
Note
Never add a permanent swap file to the system due to the performance impact on the file
system layer.
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:
RAM
1 GB - 2 GB
2 GB - 8 GB
Greater than 8GB
Table 6.1. Recommended Swap Space Requirements for 10g R2
Usage". Depending on the size of your swap space,
Swap Space
1.5 times the size of RAM
Equal to the size of RAM
0.75 times the size of RAM
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