Paging Space Tips

This document contains tips for allocating paging space on an RS/6000. Too much paging space is wasted disk space, while too little paging space causes a number of problems.

The information contained in this document is valid for AIX 3.2 through 4.x.

There is no "right" amount of paging space for a system. Paging space requirements are unique for each system, depending on the applications that are running, the number of active users, and other factors.

There are several general rules that can help determine how much paging space is needed:

NOTE: The following guidelines apply to some computing environments, but may provide too much or too little paging space for others:

• For systems with less than 64MB of RAM, the installation process creates paging space equal to two times (2X) the memory:

     2 * RAM
  

• For systems with 64MB to 256MB of RAM the following paging space equation applies:

     Page Space = RAM size + 16MB
  

• For systems with more than 256MB of RAM, use the following equation:

     Page Space = 512 + ( RAM - 256 ) * 1.25 
  

Systems with large amounts of memory typically do not need such large amounts of paging space. In a persistent storage environment, in which the machine hosts a few small programs and a large amount of data, the system may need less than one times (1X) its RAM size for paging space. For example, a 1GB database server that runs on a RS/6000 with 256MB of RAM and uses only 50MB of "working" storage does not need 256MB of paging space, or even 512MB of paging space. It needs only the amount of paging space that allows all the working storage to be paged out to disk, because the 1GB database is mostly persistent storage and requires little or no paging space.

The "two times RAM" rule is a good starting point for systems with 256MB of RAM or less. Use the following equation for systems with more than 256MB of RAM:

   Page Space = 512 + ( RAM - 256 ) * 1.25 

If a disk drive containing an active paging space logical volume is removed from the system, the system will crash.

• Do not put more than one paging space logical volume on a physical volume.

  All processes started during the boot process are allocated paging space on the default paging space logical volume (hd6). After the additional paging space logical volumes are activated, paging space is allocated in a "round robin" manner in 4KB chunks. If you have paging space on multiple physical volumes and put more than one paging space on one physical volume, you are no longer spreading paging activity over multiple physical volumes.

• Avoid putting a paging space logical volume on the same physical volume as a heavily active logical volume, such as that used by a database.

• It is not necessary to put a paging space logical volume on each physical volume.

• Make each paging space logical volume roughly equal in size.

  If you have paging spaces of different sizes, and the smaller ones become full, you will no longer be spreading your paging activity across all of the physical volumes.

• Do not extend a paging space logical volume onto multiple physical volumes.

  If a paging space logical volume is spread over multiple physical volumes, you will not be spreading paging activity across all the physical volumes. If you want to allocate space for paging on a physical volume that does not already have a paging space logical volume, create a new paging space logical volume on that physical volume.

• For best system performance, put paging space logical volumes on physical volumes that are each attached to a different disk controller.

Allocating more paging space than necessary results in unused paging space that is simply wasted disk space. But if you allocate too little paging space, a variety of unpleasant symptoms may occur on your system. Use the following guidelines:

• Enlarge paging space if any of the following messages appear on the console or in response to a command on any terminal:

  INIT: Paging space is low 
  ksh: cannot fork no swap space 
  Not enough memory 
  Fork function failed 
  fork () system call failed 
  Unable to fork, too many processes 
  Fork failure - not enough memory available 
  Fork function not allowed.  Not enough memory available.
  Cannot fork: Not enough space 
  

• Enlarge paging space if the average of the %Used column of the lsps -a output is greater than 80.

• Enlarge paging space if the %Used column of the lsps -s output is greater than 80.

Use the following commands to determine if you need to make changes regarding paging space logical volumes:

   iostat 
   vmstat 
   lsps