Paging Space
Paging Space is the other use of the Logical Volume Manager (LVM).
Paging Space is treated a special Logical Volume (LV) because that is what it is!
Created using different tools/commands, or different smit panels, but mirrored using standard LV tools.
Old style
On older AIX systems it used pre allocation of paging space - this is very good on small memory machines.
When you have to page you already have the exaclt spot on disk earmarked for the page and immediately start the disk I/O.
New style AIX 4.3.3 and AIX 5.1 onwards
On newer AIX versions the allocation of space is performed "on demand" - this is good for large memory systems and avoids CPU cycles alocating disk space that is never used.
Paging Space Size?
The old UNIX manuals use to state (we are taking 20 years ago here): "If you run out of paging space .... absolute mayhem is guaranteed."
You may get:
- Crashing processes at random (its is processes requesting more virtual memory and not handling the error)
- Commands fail to start (fork need memory)
- Login failure (ditto)
So the answer to the size questions is: Yes!
There is no correct answer but it is between half (for the brave) and six times memory (for the rich).
A starting point would be:
- for pre AIX 4.3.3 - start with two times memory
- for AIX 4.3.3 onwards and systems where there are no AIX users and well behaved applications (i.e. they have maximum number of sessions and processes) - start with half the memory size
- for AIX 4.3.3 onwards and systems where users log on to UNIX or can create lots of processes - start with two times memory.
Whatever you start with, watch it carefully for the first two weeks of serious use.
Forgetting, to increase paging space is a classic benchmark - problem and it is amazing that AIX carries on running when in benchmarks the writer has accidentally gone way over the top of paging space.
Initial paging Space
this is created utomatically during the AIX installation and its size is based on the machine memory size but it is usually not enough for large server workloads.
Recommendations
- Creating all paging spaces the same size
- Maximum spreading across disks
- Make sure they are all online
- Avoid shared disks likely to be moved to another system (might require a reboot)
- Don't have dedicated paging disks - when you have a peak in paging you want ALL disks to help out.
Creating paging space
To see current paging space use : lsps -a
Create paging space
- Use smit lvm
- set size
- set online "now"
- set online at next reboot too
Warning:
- lsps and create paging use different units!!
- use lsps -a -c
We recommend using more paging spaces rather than growing just the one but there is a limit to the number of paging spaces
- A paging space can be increased in size
- If the spaces are different sizes this can be used to make them all the same size.
- Paging spaces are used evenly therefore lots of paging spaces and lots of disks
ProtectingPaging Spaces
If a paging space disk fails, the system will halt, on reboot the paging space will be disabled.
If your paging space is not protected by RAID5 or a disk subsystem then you should consider mirroring.
For extra availability - mirror the paging space
Just like mirroring a logical volume (LV)
- smitty lvm + LV + Set LV + Add a copy
- or use command: mklvcopy command
Removing a Paging Space
Impossible - well it was on older AIX versions on newer version sit is possible you will have to check for your version.
- Use: smit lvm
- take Paging Space
- then deactivate it
- then remove it
If you can't remove it set to be offline at next reboot ... wait till you can reboot.
