Chapter 7. System Paging Space

7.1 Paging Space Overview

A page is a unit of virtual memory that holds 4 KB of data and can be transferred between real and auxiliary storage.

A paging space, also called a swap space, is a logical volume with the attribute type equal to paging. This type of logical volume is referred to as a paging space logical volume or simply paging space. When the amount of free real memory in the system is low, programs or data that have not been used recently are moved from real memory to paging space to release real memory for other activities.

The installation creates a default paging logical volume (hd6) on drive hdisk0, also referred as primary paging space. The default paging space size is determined during the system customizing phase of AIX installation according to the following standards:

7.1.1 Paging Space Considerations

The amount of paging space required by an application depends on the type of activities performed on the system. If paging space runs low, processes may be lost. If paging space runs out, the system may panic. When a paging space low condition is detected, additional paging space should be defined. The system monitors the number of free paging space blocks and detects when a paging space shortage exists. The vmstat command obtains statistics related to this condition. When the number of free paging space blocks falls below a threshold known as the paging space warning level, the system informs all processes (except the kernel process) of the low paging space condition.

7.1.1.1 Placement of Paging Spaces

The I/O from and to the paging spaces is random and is mostly one page at a time. The vmstat command reports indicate the amount of paging space I/O is taking place. A sample output of the vmstat command is shown in Figure 64.



Figure 69: vmstat Command Output

To improve paging performance, you should use multiple paging spaces and locate them on separate physical volumes whenever possible. However, more than one space can be located on the same physical volume.

7.1.1.2 Sizes of Paging Spaces

The general recommendation is that the sum of the sizes of the paging spaces should be equal to at least twice the size of the real memory of the machine up to a memory size of 256 MB (512 MB of paging space). For memories larger than 256 MB, the following rule is recommended:

Total paging space = 512 MB + (memory size - 256 MB) * 1.25

Ideally, there should be several paging spaces of roughly equal size each on a different physical disk drive. If you decide to create additional paging spaces, create them on physical volumes that are more lightly loaded than the physical volume in rootvg.

While the system is booting, only the primary paging space (hd6) is active. Consequently, all paging-space blocks allocated during boot are on the primary paging space. This means that the primary paging space should be somewhat larger than the secondary paging spaces. The secondary paging spaces should all be of the same size to ensure that the round-robin algorithm can work effectively.

The lsps -a command provides a snapshot of the current utilization level of all the paging spaces on a system.

7.1.1.3 Limitations of Volume Groups Having Paging Space

Avoid adding paging space to the volume groups on portable disks because removing a disk online with an active paging space will require reboot to deactivate the paging space and, therefore, cause user disruption.

Note

A volume group that has a paging space volume on it cannot be varied off or exported while the paging space is active. Before deactivating a volume group having an active paging space volume, ensure that the paging space is not activated automatically at system initialization and then reboot the system.

7.2 Managing Paging Spaces