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| | h1. 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 ! |
| | They are created using different tools/commands or different smit panels but mirrored using standard LV tools. |
| | | 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. |
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| | h3. 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. |
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| | h3. 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. |
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| | !ps2.jpg! |
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| | h3. 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) |
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| | 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). |
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| | 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. |
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| | Whatever you start with, watch it carefully for the first two weeks of serious use. |
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| | 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. |
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| | h3. 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. |
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| | h3. 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. |
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| | h3. 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 |
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| | Warning: |
| | * lsps and create paging use different units!! |
| | * use lsps -a -c |
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| | 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 |
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| | !ps3.jpg! |
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| | h3. 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. |
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| | 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 |
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| | h3. 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 |
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| | If you can't remove it set to be offline at next reboot ... wait till you can reboot. |
