IBM Certification Study Guide AIX V4.3 System Support

9.3 Paging Space Management

Paging space requirements are unique for each system depending upon the kind of applications running and/or the number of active users.

9.3.1 Do You Need More Paging Space?

If any of the following messages appear on the console or in response to a command on any terminal, it indicates a low paging space.

"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"

9.3.2 Paging Space Tips

The following tips, in general, will help in creating or increasing the paging space.

Do not allocate 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 4 KB blocks. If you have paging space on multiple physical volumes and have 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 a heavily active logical volume. For example a volume that is being 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 the best performance on a system having multiple disk controllers, allocate paging space logical volumes on physical volumes that are each attached to a different disk controller.

9.3.3 Managing Default Paging Space (hd6)

The AIX installation defaults to a paging logical volume (hd6) on drive hdisk0, that contains part or all of the busy / (root) and /usr file systems.

A special procedure is required for removing the default paging space (hd6). These paging spaces are activated during boot time by shell scripts that configure the system. To remove one of the default paging spaces, these scripts must be altered and a new boot image must be created.

9.3.3.1 Reducing the Size of hd6 Paging Space

The following example shows the steps involved in reducing the size of hd6 paging space from 160 MB to 120 MB.The steps in the following procedures are all necessary, even those not directly related to the hd6 paging space. The additional steps are needed because a paging space cannot be deactivated while the system is running

Note

AIX Version 4.2.1 and later does not support reducing the size of hd6 below 32 MB or the system will not boot.
If you decide to reduce hd6, you must leave enough space for the software in rootvg. A rule of thumb for reducing hd6 paging space is to leave enough space to match physical memory. To find out the amount of physical memory, use the following command:

lsattr -E -l sys0 -a realmem

Note
AIX Version 4.2.1 and later does not support reducing the size of hd6 below 32 MB or the system will not boot. If you decide to reduce hd6, you must leave enough space for the software in rootvg. A rule of thumb for reducing hd6 paging space is to leave enough space to match physical memory. To find out the amount of physical memory, use the following command: `lsattr -E -l sys0 -a realmem`

Create a temporary paging space on rootvg by executing the following command:
```
mkps -a -n -s 30 rootvg hdisk0
```
This command outputs the name of the paging space (paging00 if no others exist).
Use the following command to deactivate the hd6 paging spaces in preparation for the reboot later in the procedure:
```
chps -a n hd6
```
Change the paging space entry in the /sbin/rc.boot file from:
```
swapon /dev/hd6
```
to
```
swapon /dev/paging00.
```

Execute the following command to check the primary dump device designation:

# sysdumpdev -l
primary              /dev/hd6
secondary            /dev/sysdumpnull
copy directory       /var/adm/ras
forced copy flag     TRUE
always allow dump    FALSE

If the primary dump device is hd6, change it to some other paging space. The following command shows how to change the primary dump device to paging00:

# sysdumpdev -P -p /dev/paging00
primary              /dev/paging00
secondary            /dev/sysdumpnull
copy directory       /var/adm/ras
forced copy flag     TRUE
always allow dump    FALSE

Create a bootable image with the bosboot command for a hard disk image:
```
bosboot -d /dev/hdisk0 -a
```
Put the system key (if present) in the normal position and use the following command, which will both shutdown the operating system and reboot it:
```
shutdown -r
```
After the system reboots, remove the hd6 paging space:
```
rmps hd6
```
Create
a new paging space logical volume of the size 120 MB for the hd6 paging space:
```
mklv -t paging -y hd6 rootvg 30
```
Change the primary dump device designation back to be the paging space hd6:
```
sysdumpdev -P -p /dev/hd6
```
Change the paging space entry in the /sbin/rc.boot file from
```
swapon /dev/paging00 to swapon /dev/hd6.
```
Create a bootable image with the bosboot command for a hard disk image;
```
bosboot -d /dev/hdisk0 -a
```
Execute the following command to make the new hd6 paging space automatically activated when the system reboots:
```
chps -a y hd6
```
Execute the following command to change the attribute of temporary paging space, paging00, so that it does not automatically activate after next reboot:
```
chps -a n paging00
```
Put the system key (if present) in the normal position and use the following command, to shutdown and reboot the system:
```
shutdown -r
```
After the system reboots, remove the temporary paging space:
```
rmps paging00
```

Use the lsps -a command to verify the reduced size of default paging space (hd6).

9.3.3.2 Moving the hd6 Paging Space to Another Volume Group

Moving a paging space with the name hd6 from rootvg to another volume group is not recommended because the name is hard-coded in several places.

Only the paging spaces in rootvg will be active during the second phase of the boot process, and having no paging space in rootvg could severely affect system boot performance. If you want the majority of paging space on other volume groups, it is better to make hd6 as small as possible (the same size as physical memory) and then create larger paging spaces on other volume groups.

9.3.3.3 Moving the hd6 Paging Space within the Same VG

Moving the default paging space from hdisk0 to a different disk within the same volume group does not require system reboot.

The following example shows the command used to move the default (hd6) paging space from hdisk0 to hdisk1:

migratepv -l hd6 hdisk0 hdisk1

This may take a few minutes depending upon the size of paging space.

9.3.4 References

The following publications contains more information about system performance tools.

AIX Version 3.2 & 4 Performance Tuning Guide, SC23-2365
AIX Version 4.3 Messages Guide and Reference, SC23-4129
URL: http://service.software.ibm.com/rs6k/techdocs

9.4 Quiz