AIX 5L SysAdmin I: (Unit 09) – Paging Space

Objectives:
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1. Define why paging space is required in AIX
2. List and monitor the paging space utilization of the system
3. Perform corrective actions to rectify too little or too much paging space scenarios

Paging Space:
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Is a secondary storage area for over-committed memory
Holds inactive 4K “pages” on disk
Is NOT a substitute for real memory

Paging Space is for memory overcommitment.
Takes the inactive parts of memory and writes them out to disk in 4K chunks.

Note: If you have too much paging going on then you are going to have very poor performance.

Sizing Paging Space: (Logical Volume: hd6)
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Created at installation up to twice the size of real memory
Amount needed is dependent on applications
Monitor paging space: lsps -a
Running low on paging space is bad

#
ksh: cannot fork no swap space

Note: When there is not enough paging space, the system will determine the least active processes and start to kill those processes off. This helps keep the system up and running instead of allowing it to crash.

Paging Space Placement:
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Only one paging space per disk
Use disks with the least activity
Paging spaces roughly the same size
Do not extend paging space to multiple PV’s – Physical Volumes
Use multiple disk controllers
hd6 – Created during installation
paging00
paging01

Note: Must run the lsps -a command often throughout the day to determine the correct amount of paging space that is needed since the reported space used fluctuates significantly depending upon what is being run on the system.

Total RAM:
lsattr -El sys0 -a realmem
realmem 262144 Amount of usable physical memory in KB Fales

Paging Space Activated at startup:
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cat /etc/swapspaces
hd6: dev=/dev/hd6
paging00: dev=/dev/paging00

Adding Paging Space:
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# smit mkps

Change Paging Space:
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# smit chps

Note: Since you can’t reduce the size of a logical volume, this command will create the new logical volume for the paging space, move the data from the current paging space to the new paging space, and the remove the old logical volume. The remove partitions option is new to AIX version 5.0.

Remove Paging Space:
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In order to remove an active paging space:
1. Make inactive
# swapoff /dev/paging00

2. Remove inactive paging space
# rmps paging00 (Can be done through SMIT)

Note: /dev/hd6 cannot be removed using this process.
It is more difficult to change paging space hd6. Check the internet or IBM support.

Problems with Paging Space:
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1. Paging space too small:
Dynamically increase the size by allocating more partitions
Can also create additional paging space

2. Paging space too larg (only for user-created paging space):
Decrease paging space
# chps -d LogicalPartitions PagingSpace

Example:
# chps -d 2 paging00

Documenting Paging Space Setup:
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Run the lsps command
Have a hardcopy of the /etc/swapspaces file

FAQs:
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Q: How do I prevent thrashing?
A: Make sure you have enough RAM.
?????
Q: Won’t increasing the paging space help with thrashing?
A: No. Thrashing results because there is not enough RAM. As a result, the OS pages out part of RAM to the paging area. When too much needs to be paged out because memory is overcommitted, thrashing occurs. So the problem isn’t paging space, it is RAM.

Q: I want my paging space in the center, but when I created the paging space, it didn’t give me an opportunity to set policies. How do I do that?
A: Paging is just a logical volume. Go in to the LV path on SMIT and change the policies just like on any other LV. Don’t forget to reorganize the VG to move the partitions.

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