How to deal with Paging Space in AIX?

How to deal with Paging space in AIX?

PAGING SPACE TIPS:

Determining if more paging space is needed:

Allocating more paging space than necessary results in unused paging space that wastes disk space. However, allocating too little paging space can result in one or more of the avoidable symptoms listed below.

Use the following guidelines for determining the necessary paging space:

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

ü INIT: Paging space is low

ü ksh: cannot fork no swap space

ü Not enough memory

ü Fork function failed

ü fork () system call failed

ü Unable to fork, too many processes

ü Fork failure - not enough memory available

ü Fork function not allowed. Not enough memory available.

ü Cannot fork: Not enough space

Add a paging space if the average of the %Used column in the output of the lsps -a command is greater than 80

Add a paging space if the %Used column in the output of the lsps -s command is greater than 80.

Note: Only extend a paging space as a last option.

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

Iostat:  Check the tm_act field for the hdisk containing the paging space for a high percentage relative to the other hdisks

vmstat : Assure fr/sr columns of the vmstat page field do not consistently exceed the ratio of 1:4.

lsps : Use the -a flag to list all characteristics of all paging spaces. The size is given in megabytes. Use the –s flag to list the summary characteristics of all paging spaces. This information consists of the total paging space in megabytes and the percentage of paging space currently assigned (used). If the -s flag is specified, all other flags are ignored.

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THINGS TO CONSIDER WHEN CREATING OR ENLARGING PAGING SPACE:

Before creating a new paging space or enlarging an existing paging space, consider the following:

   Ø If a disk drive containing an active hd6 paging space logical volume is removed from the system, the system will crash.

   Ø Do not put more than one paging space logical volume on a physical volume.If you add more than one paging  space to one of the physical volumes, the paging activity is no longer spread equally across the physical volumes.

   Ø All processes started during the boot process are allocated paging space on the default paging space logical  volume (hd6). When additional paging space logical volumes are activated, paging space is allocated in a "round robin" manner, in 4KB chunks.

   Ø 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.

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

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

   Ø If paging spaces are of different sizes, and the smaller ones become full, paging activity will no longer be spread across all of the physical volumes.

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

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

   Ø It is technically supported to create default paging space (hd6) on ESS, EMC or RAID array, although it is not recommended, and should be avoided if possible.

   Ø NOTE: If system is paging enough to cause an I/O bottleneck, tuning the location of the paging space is not the answer.