HP-UX System Administrator's Guide: Overview

File System Swap Guidelines
When you need more swap space and you have no devices available for additional
device swap, or if you need to swap to a remote system, you can dynamically add file
system swap to your system. Use the following guidelines:
• Interleave file system swap areas for best performance.
Two swap areas on different disks perform better than one swap area with the
equivalent amount of space. Multiple devices allow for interleaved swapping which
means the swap areas are written to concurrently, minimizing disk head movement,
thus enhancing performance. This applies as much to file system swap space as it
does to device swap space, so the same guideline applies.
To see which devices are already being used for file system swap use the command:
swapinfo -f
• If possible, avoid configuring heavily used file systems. Heavily used has two
meanings here:
1. Actively used file systems (for example, the root file system, or those file systems
used most frequently by your primary applications). This will slow down the
performance of your server as paging activities compete with your applications
and user file access.
2. Very full file systems. Because file system swap uses unused space within file
systems, if the file systems are very full there is not much unused space for
paging use (and it is probably very fragmented within the file system). To
gauge how full a file systems are, use the bdf command.
Guidelines for Assigning Swap Priority
When you add swap areas, you can assign a priority to each. Priorities range from 0
(the highest) to 10 (the lowest). HP-UX uses swap areas with higher priority first. And,
HP-UX gives device swap priority over file system swap when each has the same
priority. Here are the guidelines you should use:
• Given multiple swap devices with identical performance, assign each an identical
priority. By so doing, you will allow the system to use each of them on an
interleaved basis which enhances performance.
• Assign higher priorities to the swap areas that have faster performance and lower
priorities to areas that are slower.
• Do not give file system swap areas priority over device system swap areas.
Although this is not absolutely necessary it enhances the swapinfo output.
• Give lower use file systems priority over higher use file systems.
For More Information on Configuring and Managing Swap Space
The following manpages contain important information on configuring swap space:
74 Major Components of HP-UX